I found Part 1 of Leah's journal in her room of the Slaughtered Calf Inn. Where do I find the remaining parts? And how many parts are there?
Leah has 16 journals. They respawn in the same places in the first three acts, new journals taking the previous journal's place (so get them while you can). You can return to previous chapters if need be.
Act 1's journals 1-7 are on a table in Leah's Room, a room in the back of the Inn.
Act 2's journals 8-12 are on a table in the east of in the town.
Act 3's journals 13-16 are in the southeast room of the Armory (the room in the town's southeast corner with Adria/Leah)
There are many lore books. You can check your book collection progress from the achievement list (press Y). Leah's journals are listed under
There are 16 parts of Leah's Journal. They are found in the same location in each of the town hubs in Acts I-III. The specific parts are available immediately from the start of the first listed quest (it must have been started and listed as your objective) until the end of the last listed quest (no longer listed as your objective) in the list to follow.
Parts 1-7 are found in the back room of the tavern in New Tristram in Act I:
Parts 8-12 are found on a table to the south-east of the Hidden Camp in Caldeum in Act II:
Parts 13-16 are found on a table to the south-east of The Armory in Bastion's Keep in Act III:
You can check which ones you've seen across all your characters by checking the People Lore achievements A Quick Study (Campaign Act I), Judge of Character (Campaign Act II) and Primary Sources (Campaign Act III). Each character records lore separately so the only ways to know if a specific lore has been read by a given character (for the XP bonus if not for being thorough) is to check for it in the lore screen or to try reading the specific lore.
Leah's journal is tied to how far you are along Act 1.
Each time you complete a major quest, go back to her room to find another page of her journal.
Cain has journals within his house in New Tristram as well.