It's got 2.8m members who have entered 186m books that they own. It cross-references those collections in some way (I don't know anything about the internals).

It's got 2.8m members who have entered 186m books that they own. It cross-references those collections in some way (I don't know anything about the internals). I just checked, and it says it's recommending a book to me (Singularity Sky) based on five other books I own (Engine City, Spin State, A Deepness In The Sky, Recursion, The Skinner). But as it's always "based on these five books", I have to assume it's doing a lot more matching than that.