Gemara

(Rabah and Rav Yosef): We may not read from a Sefer Haftorah (a scroll containing only the passages from Nevi'im read for the Haftorah), for one may not write such a Sefer. (One may write only complete Sefarim of Tanach.)

(Mar bar Rav Ashi): One may not even move it on Shabbos, for one may not read from it!

Rejection: The Halachah does not follow these opinions. Rather, we may move and read from such a Sefer. We learn from R. Yochanan and Reish Lakish, who read from a Sefer of Agadata on Shabbos.

Question: One may not write oral Torah!

Answer: Since there was no alternative (the teachings were in danger of being forgotten), they could be written - "Es La'asos la'Shem Heferu Torasecha."

Similarly, we may write and use a Sefer Haftorah (since not every Tzibur can afford a complete set of Nevi'im).

Megilah 24a (Mishnah): We may skip while reading in Nevi'im, but not in Torah. One may skip only a small amount, so that he can roll the Sefer to the new place before the translator finishes translating.

(Beraisa): In Torah we may skip within a subject, and in Navi to another subject.

(Beraisa): We may skip from one Navi to another only within Trei Asar (the 12 minor) Nevi'im, and only in forward order.

Rishonim

The Rif and Rosh (5:20) bring the Gemara in Gitin verbatim.

Question: Some places have a custom, when a Chasanah occurs in one of the four Parshiyos or Chanukah, to read some Pesukim of Sos Asis, and then to skip to the Haftorah for that Parshah. We may not skip from Navi to Navi!

Answer (Terumas ha'Deshen 20, brought in Beis Yosef OC 144 DH Medalgin): The Mordechai cites Avi ha'Ezri to say that the Isur to skip b'Tzibur is due to the honor of the Tzibur, like the Isur to roll a Sefer Torah b'Tzibur. This was when Sefarim were written like our Sifrei Torah, and there was a delay to roll it from one Navi to another. Our Haftorah is written in books. One can leave a marker and quickly find the desired place. A Tzibur with only one Sefer Torah rolls it on Yom Tov, or when Shabbos is Rosh Chodesh or one of the four Parshiyos (the Maftir is in a different Parshah than the reading for that Shabbos). There is no problem of Kavod Tzibur when different people read and we roll in between. However, Rashi says that we do not skip lest this disorient people. We may skip within one Navi, for we learn few Halachos from it, and this does not disorient so much if it is less than the time to translate the verse. According to this, it does not depend on how the Navi is written! However, since we do not translate nowadays, there is no concern for disorientation. Even the days of the Gemara, they did not translate in all places. The Isur to skip was only where they translated. The Shi'ur for skipping was based on the time to translate! According to Rashi, we cannot justify skipping in places with only one Sefer Torah. Where the custom is to read the Haftorah from books, they may continue to do so. Where this is not the custom, they should not begin to do so.

Beis Yosef: R. Mano'ach says that since we read Sos Asis by heart, this is not considered skipping. Mahari ben Chaviv permits because we do not read it l'Shem Haftorah, rather, like a song.

Machatzis ha'Shekel (OC Reish Siman 284 DH v'Efshar): Nowadays we do not write Nevi'im on rollers, because it is our custom to skip, and we cannot roll the Sefer (quickly enough).

Poskim

Shulchan Aruch (OC 284:1): We read in the Navi a matter related to the Parshah.

Beis Yosef (DH Kasuv bi'Teshuvos): The Kol Bo (20) cites the Rashba to say 'it is impossible that we are not particular, and we read the Haftorah with a Berachah even without a Sefer.' The text should say 'it is possible.' I.e., as long as we read the Torah, even without a Kosher Sefer Torah, we bless on the Haftorah.

Magen Avraham (Reish ha'Siman) and Machatzis ha'Shekel (DH Perush): The letters 'Aleph Aleph' in the Kol Bo stand for 'I say', not 'impossible' (Bach DH bi'Teshuvas). The Kol Bo and Rashba discuss blessing on a Navi not written like a Kosher Sefer, unlike the Beis Yosef explained. The Rashba (487) says that we are particular to bless on Kri'as ha'Torah only if there is a Kosher Sefer. He does not say so about the Haftorah.

Levush (1 and 559:1): Since Kri'as ha'Haftarah is only b'Tzibur, I am astounded that the custom is not to write it according to the laws of a Sefer, like we write the Megilah. It seems that one is not Yotzei through printed Chumashim. Likewise, Megilas Eichah should be written on a Klaf, for the Tzibur must hear it! Perhaps we do not, lest it show despair from Ge'ulah.

Taz (2): He overlooked the Gemara in Gitin. Rashi explains that one may not write less than a full Sefer. Therefore, even if the Haftaros were written, they would not be a Sefer. It is permitted only due to "Es La'asos la'Shem." The same applies to printed Chumashim.

Magen Avraham (ibid.): The Heter for Sefer Haftorah was because not every Tzibur can afford a complete set of Nevi'im. Nowadays that Seforim can be printed cheaply, one may not write Haftaros, for printing is considered writing. Shiltei ha'Giborim (Shabbos 43a DH Kol) says that only a Sefer Torah or Megilah must be written with ink. Likewise, paper suffices in place of parchment (Tosfos Shabbos 115b DH Megilah). It need not be on rollers; we bless on Eichah and Shir ha'Shirim even though they are not on rollers.

Machatzis ha'Shekel (DH d'Ha): The Taz and Magen Avraham teach that now that printed Nevi'im are cheap, "Es La'asos la'Shem" does not apply.

Korban Nesan'el (5:4): According to this, nowadays one may not write Haftaros (even) on parchment, since they are not a full Sefer; they are Muktzah. This requires investigation.

Eshel Avraham (Reish ha'Siman): It is better to read from a full Tanach than from the Haftorah printed by itself in a Chumash.

Mishnah Berurah (1): The Taz and Magen Avraham permit reading the Haftorah from something printed. The Magen Avraham and Eliyahu Rabah require a full Navi. Haftoros in a printed Chumash do not suffice. However, if there is no full Navi, one may be lenient, and not omit the Haftorah. L'Chatchilah, every Tzibur should have Nevi'im written properly on parchment, for then Hash-m's name is written bi'Kedushah. The Gra enacted so in his congregation. Fortunate are those who have adopted this.

Sha'ar ha'Tziyon (1,2): It is better to read the Haftorah from a printed Chumash than from a Tanach written by heretics. The Eshel Avraham discusses reading from a Tanach. Really, a full Navi suffices.

Kaf ha'Chayim (3): The custom is to read from Chumashim. Some places write Nevi'im on parchment.

Kaf ha'Chayim (5): One may read even if the Kri (the tradition how to pronounce a word) is written in the text, and the Kesiv (how it is written) is in the margin. L'Chatchilah one may not do so write it this way.