Tucker basically only reads books about vehicles (his recent favorites are about race cars), but occasionally we hit upon another subject that will hold his attention.
We've found a series of "Tom and Pippo" books by Helen Oxbury that Tucker loves. They feature a little boy and his stuffed monkey doing various mundane things (...make a mess, ...see the moon, ...go shopping, etc.). They are sweet, very short (one of my criteria for a good naptime book!), and Tucker is obsessed.
So, if you have a little boy and are dying to read anything besides trucks, trains and cars... try Tom and Pippo. At this point, unfortunately, I have read these books so much that the car books are starting to look pretty good again to me... maybe this is the point?