I'm thinking of maybe using wikispaces, but I don't know if there's other stuff out there...

I love Wikispaces, but Google is also a good way to go. Wikispaces will let you create student accounts without emails if that's an issue

That is one perk of wikispaces, which I'm thinking about using now, but only one person can work on it at a time, right?

only one person should edit a page at a time. But, if you give each student a separate page, you won't have that problem.

Don't rule out adding a shelfari widget to the wiki page. Kids can do reviews right there too. It's good. See? bit.ly/aVRXWf

Mine's in the sidebar but kids can add them to their own pages too.

I think a cool thing for them to do might also be to create a voicethread about their book. I haven't done that yet. Maybe someday.