Forums are easy, you don't need books for it (assuming you know PHP & mySQL already). First download existing forums and check the source code. Also install it and check how the database is build (via phpMyAdmin). It will give you tons of tips.
The database is basicly like that:
- user table (you put login/password here, also user/moderator/admin status)
- board table (for boards/subboards)
- topic table (store the information of each topic)
- post table (store the information of answers/replies for each topic)
The core modules are:
- register
- login/logout
- display the forum (index page, subboards, threads list, thread)
- post new thread, answer old thread, edit old post, delete your post/thread
- admin/moderator area (create/edit/remove new board, change user status, ban user)
- instalator