In general, just things that interest me. Especially things that are "controversial" in some sense.
My first book was about database design, which may not sound like a controversial subject -- there have been relatively few street riots over disagreements about database design -- but I think many people do it very, very wrong, and so I wrote the book to say how I think one should do it right.
My second book was about defending the Bible from atheist's attacks. More obviously controversial.
My third was about Heaven. The ideas that most people have about Heaven are very different from what the Bible says. Like nowhere does the Bible say that you become an angel when you die or that in Heaven you have wings and float around on clouds. Even Bible-believing Christians tend to have a vastly over-simplified view of Heaven.
And my fourth book was about end-time prophecy. In that book I tried to very explicitly discuss all the competing theories and the evidence for each.