I'vRead is a record of what books a user has read. Users don't need to sign up for an I'vRead account, they only have to post a Tweet in the format 'book name @ivread comment' and it will update their I'vRead record. Other users can then see lists of what other users have read or lists of popular books. The I'vRead API is a way for developers to integrate I'vRead into their own applications/websites. The API can be used to get JSON or RSS data about Recent Reads or User Information.
I'vRead is a service for keeping track of what books you've read. Seems simple, but it can be rather useful for those of us obsessed with reading like myself. Its web site offers the service for free, and using it is already pretty simple. There isn't even another account to register for. All a user needs to do is add a specific tag, @ivread, to a Twitter post mentioning the book. It allows for some basic searches of the data on their website, but the I'vRead API is where the service really shines.