The word for “book” is always pronounced ほん (and written 本). The kanji is occasionally read もと when it’s a part of some other word, but it looks like only in a small number of fairly obscure words. In those cases it seems to be largely being used with its other meanings of “origin”, “main”, etc, not as anything book related.
(The word もと meaning “origin” etc is a common one, but it is written with 元.)
The latter. Also needs a を in there if you want to avoid being too casual.
You’ll frequently find 本 using its kun’yomi in family names, like 橋本 = はしもと. In place names, though, it could be either, just to be tricky, especially when it’s a prefix - compare 本銚子 (もとちょうし) and 本駒込 (ほんこまごめ).