Every Shabbat and Yom Tob, the congregation conducts a Haftara reading from the Prophets after the Torah reading. What is the preferred book that a congregation should use for this reading?

Ideally, the congregation should use what we call a "Sefer Haftarot," a parchment containing all the Haftarot readings. This parchment does not contain the entire Prophets section of the Tanach; it includes only the Haftarot. Nevertheless, it is the most preferable scroll to use for the Haftara reading.

If no such scroll is available, then a congregation should use a printed Tanach, a book containing the entire Torah, Prophets and Ketubim. Indeed, most synagogues keep a large edition of the Tanach near the Bima (table) and use it for the Haftara reading. This is the preferred practice if a Sefer Haftarot parchment is not available.

If the congregation does not even have a printed Tanach, then the Haftara should be read from any Humash where the Haftara is printed. However, although this practice does have ancient origins and is grounded in Halachic sources, the more preferred practice, as mentioned, is to read from a Sefer Haftarot parchment, or at least from a printed Tanach.

Summary: Ideally, the Haftara should be read from a "Sefer Haftarot" parchment, which contains all the Haftarot. Otherwise, it should be read from a printed Tanach. Only if these are not available may the Haftara be read from an ordinary printed Humash.