Read-a-long forum
Members of the forum read the same book at the same time and discuss it.
Warning: discussions here will contain spoilers.

Personal Reading Diaries
Members of the forum can keep notes about their reading. You can post your notes and opinions about what you're reading. Kind of like a personal reading blog, but in thread format, and where other members would be able to read and interact with you. And, of course, you would be able to read and interact with other members.