Gemara

(Beraisa): Even though Chachamim forbid reading Kisvei ha'Kodesh, one may expound them. If one needs to see a verse, he may bring the Sefer and read it;

R. Nechemyah says, they forbade reading Kisvei ha'Kodesh in order that people will reason -- if one may not read Kisvei ha'Kodesh, all the more so one may not read Shitrei Hedyotos (commoners' documents)!

148b (Mishnah): One may count his guests and courses by heart, but he may not read [from a list].

Question: Why is it forbidden?

Answer #1 (Rav Bivi): This is a decree lest he erase.

Answer #2 (Abaye): This is a decree lest people read Shitrei Hedyotos.

Objection: Also Rav Bivi should be concerned for reading!

(Beraisa): One may count which guests he will seat inside and which outside, and which courses have not yet been served, reading from [writing on] a wall, but not from a tablet or ledger.

If it is written in ink, what is the difference? (In every case he might erase!) Rather, they are etched. (This shows that that even on a tablet, one may not read Shitrei Hedyotos.)

(Beraisa): One may not read a caption under a picture or portrait on Shabbos.

Rishonim

Rosh (23:1): The Halachah does not follow Rav Bivi. One may read writing on the wall, but not on a tablet. The latter can be confused with Shitrei Hedyotos. The Ri says that Shitrei Hedyotos are documents of sale, loans, and securities. They are forbidden due to "mi'Mtzo Cheftzecha v'Daber Davar." If one reads a list of guests and courses, he might come to read Shitrei Hedyotos. Presumably, we similarly forbid Igros Reshus (letters not about Mitzvos). We could distinguish and say that we decree only about these (list of guests and courses), for they are like calculations and needs. They resemble Shitrei Hedyotos more than letters of Reshus, which are greetings, do. However, since we forbid captions under pictures and portraits, for which there is no need, due to Shitrei Hedyotos, the same applies to Igros Reshus.

R. Yonah (cited in Rosh, ibid.): There is a proof from 116b. R. Nechemyah forbids Kisvei ha'Kodesh to show people that Shitrei Hedyotos are forbidden. Surely he forbids Igros Reshus for the same reason. We learn from him to Rabanan. Rabanan argue only about Kisvei ha'Kodesh. They hold that this reason does not justify forbidding Kisvei ha'Kodesh. Igros Reshus can be confused with Shitrei Hedyotos. Some say that they forbade only reading lists aloud, but one may read silently. I forbid reading even silently. The Tosefta forbids looking at captions. One may not read riddles and parables of secular matters and passion on Shabbos. Even during the week, who permitted it?! It says "uv'Moshav Letzim Lo Yashav"!'

Tosfos (116b DH v'Chol): Rashi says that Shitrei Hedyotos are Igros Reshus. This is difficult, for the custom is to read them! The Ri says that Shitrei Hedyotos are loan documents and similar matters. One may read letters, for sometimes they contain matters of Piku'ach Nefesh. R. Tam permits even if one knows that there is no matter of Piku'ach Nefesh inside. It is not Shitrei Hedyotos, since he does not need the contents, for he already knows what it says. If he does not know, perhaps there is a great need or Piku'ach Nefesh! The Yerushalmi connotes like this. It forbids Kisvei ha'Kodesh due to Shitrei Hedyotos. If we would permit, one will say 'what is the problem if I engage in my documents?!' This implies that we discuss only matters such as loans and [other monetary] documents. Rashi says so below regarding the decree lest one read Shitrei Hedyotos. However, Rabbeinu Yehudah forbids war chronicles written in other languages. They are no worse than captions. One may not read them on Shabbos. Even during the week, the Ri did not know who permitted them. This is Moshav Letzim!

Poskim

Shulchan Aruch (OC 307:16): On Shabbos one may not read riddles and parables of secular matters and passion, e.g. Emanuel or war chronicles.

Mishnah Berurah (58): We do not forbid Yusifun (chronicles covering the time of the Churban), Sefer Yuchsin (lineage), Divrei ha'Yamim of Ri ha'Kohen and Shevet Yehudah, for one learns from them Musar and Yir'as Shamayim. Therefore, they are permitted even in other languages.

Kaf ha'Chayim (111): One may learn Yusifun, Sefer Yuchsin and Tzemach David on Shabbos, but he should not fix times for them, due to neglect of learning Torah.

Kaf ha'Chayim (112): It is good to learn the sixth Sefer of Yosifun between the 17th of Tamuz and Tish'ah b'Av, but not on Shabbos.

Kaf ha'Chayim (113): If there is war now, sometimes one must read about the current war to know what is happening. She'alas Ya'avetz (1:162) permits also if one is pained not to know what is happening. However, marketing information is printed at the end. One may not think about business based on what he reads. One cannot be careful to read only part, especially for one who sells Seforim. We forbid him. If others are stringent, they will be blessed.

Shulchan Aruch (ibid.): Even during the week it is forbidden due to Moshav Letzim.

Magen Avraham (22): The same applies to going to theaters and stadiums and Tachbulos (Chayei Adam - this is sleight of hand magic tricks). Who permitted this on Purim?! Perhaps this came from the mockery they do to recall Achashverosh.

Mishnah Berurah (59): The only frivolity permitted on Purim is the mockery of Achashverosh. In our many sins, some people freely go to theaters. It says "Al Tismach Yisrael Al Gil..." (Yisrael should not rejoice the way Nochrim do)! This also incites the Yetzer Ra. Anyone who is frivolous falls to Gehinom, and afflictions beset him - "Al Tislotzatzu Pen Yechzeku Mosereichem" (Avodah Zarah 18b).

Shulchan Aruch (ibid.): He transgresses "Al Tifnu El ha'Elilim" - Al Tifnu El mi'Daitchem.

Mishnah Berurah (60): This means "do not divert your mind to your whims."

Magen Avraham (23): Tosfos (149a DH v'Diyoknei) and the Rosh (23:2) say that one transgresses only for portraits made for idolatry. However, in Avodah Zarah (50a DH Hachi), Tosfos forbids regarding all portraits, except for the form of a coin, which one always sees. Veering does not apply to it. People conduct like the first opinion. It seems that all permit merely seeing it. See 229:1. (We bless on seeing a rainbow, but one may not look intently.)

Bi'ur Halachah (DH v'Over) Also the other Poskim permit portraits for beauty, like Tosfos and the Rosh. Some are stringent. Even they forbid only looking intently.

Shulchan Aruch (ibid.): There is an additional Isur of passion, due to inciting the Yetzer Ra. The author, the one who copied it, and all the more so the printer, make the Rabim sin.

Mishnah Berurah (61): This includes people who sell these books and make them available to buyers. One who causes another to sin is worse than one who kills him. Killing is [harmful to the victim] only in this world, but causing him to sin ruins also his world to come.

Bi'ur Halachah (DH Machti'im): One who causes another to sin is not given opportunity to repent. One who scoffs, his food is diminished, afflictions come upon him, and he falls to Gehinom. Surely the same applies to those who cause hundreds to scoff!

Rema: We forbid secular matters and war chronicles only in other languages. They are permitted if they are in Lashon ha'Kodesh. The custom is to be lenient.

Taz (13): Rabbeinu Yehudah forbids war chronicles written in other languages. They are no worse than captions under pictures and portraits. It seems that he means that they are not no better than captions. The Rema inferred that it is permitted in Lashon ha'Kodesh. There is no proof for this. Tosfos mentioned other languages to clarify which war chronicles he discusses. The language itself has no Kedushah. One may speak secular matters in a bathhouse in Lashon ha'Kodesh (Shabbos 10b). Also, it is Moshav Letzim even in Lashon ha'Kodesh! Frivolity does not depend on the language! Also the Bach argued with the Rema. This is primary. Surely, on Shabbos one may not move anything that one may not read. The Beis Yosef brought so from Teshuvas ha'Rashba regarding a telescope.

Magen Avraham (24): The language itself has Kedushah. One learns Divrei Torah from it. One may move a Get even if it is written in other languages, for one can learn laws of Gitin from it. However, the Shlah and Agudah forbid. The Bach forbids even in Lashon ha'Kodesh. The Rema permits even a friendly letter in Lashon ha'Kodesh, for one can learn Halachos from it, and it contains several Torah verses.

Mishnah Berurah (63): Even the Rema forbids loan documents and calculations even in Lashon ha'Kodesh. Shevus Yakov is lenient about reading a newspaper on Shabbos, but many Acharonim forbid, for it includes many business matters.

Mishnah Berurah (64): Many Acharonim argue with the Rema. Also Igros Reshus do not depend on the language.