This community is for exactly what the name suggests. When you're saying to yourself, "What was that book I read back in college/high school/grade school (or some other time frame)? I know there there was a mother and a father and kid with x condition, and so on..." Maybe you want to read the book again, or maybe you want to buy it for a friend or relative. This gives you a place to go for help.
I posted a book description to the community and I had an answer, and the correct answer at that, within 12 hours. So now I've gone back and looked at other people's requests. I thought I read a lot, as a child and a twenty-something, but I didn't recognize more than one or two descriptions out of dozens. On the other hand, the other folks in the community seem to have about a 50 percent hit rate, although it's tough to say for sure. There are also many requests for titles of books that certainly sound like speculative fiction, so that's encouraging, too. I'd be interested in knowing how the folks on my friends' list compare, so if you have a mo, check them out and let me know in a comment how you did.
Now if they just had an LJ community called "What was the name of that LJ community?" -)