Brian Weatherson links to the Unsuggestor, which uses Amazon personal profiles to match up books people have with books they're not likely to have. It's sort of the inverse of Amazon's engine for recommending books based on what other people who bought what you bought have bought. I tried a few books I've got, and I discovered some disturbing things. Consider the following sets of unrecommendations:
They have the second Harry Potter book opposed to The Gospel According to John, by Leon Morris, a fairly respected evangelical commentary on the fourth gospel. I have both books and like them both very much. Most of the Harry Potter books have several John Piper books turning up in the top five, mostly some of his newer books (which I don't have), but his earlier Desiring God turned up with some of novels by Terry Brooks, one of my favorite fantasy authors. This would again be a case of two books I pretty much like (even if I criticize Piper on a few issues here and there). Some books in Terry Pratchett's Discworld series are put up against John Piper, Josh Harris, Wayne Grudem, A.W. Tozer, J.I. Packer, and other books by evangelicals, including several books I've got or have at least spent time looking through. Pratchett's Reaper Man isn't my favorite of the Discworld series, but a lot of it is funny. Its opposite is Doug Stuart and Gordon Fee's How to Read the Bible for All Its Worth, one of the best popular introductions to biblical interpretation ever written. Pratchett's much better Lords and Ladies is opposed to Knowing God by J.I. Packer, one of the most important popular introductions to theology in print. While I don't think Grudem's Systematic Theology is well-argued on the level of detailed exegesis (as in the classic tradition of Reformed systematic theologies like Hodge's), it's an excellent reference work, and I think his positions are largely correct on most issues. It's opposed to Pratchett's Pyramids, a Discworld book I very much loved. D.A. Carson's guide to New Testament commentaries, something I use all the time, lists Harry Potter book 6 as its opposite, a book that is next on my list to read. Carson's How Long, O Lord?, the best book I've seen on the problem of evil, also lists Potter book 6 as its first unsuggestion.