We initially got this book out of the library but my son (21 months) loves it so much that (having already renewed it twice!) I'm buying our own copy. On the first couple of readings, the appeal was simply a book with a whole double-page spread of different cars, another with a whole page of different trains etc. but as his vocabulary increases, it has become great for discussing and comparing shapes, sizes, colours etc.
It's very much the kind of book where you talk about the pictures with your child, rather than just go through reading the text. There are also some useful pages at the end that give you ideas and things to hunt for if inspiration runs dry.