I've seen a few recommendations on other threads and I thought it may be a good idea to collate them all in once place.
So if you'd like to recommend a book to another step-parent - add it here!
I have two recommendations,
Google Eyes & Step By Wicked Step. Both by Anne Fine.
These are both children's books, but if you've ever read Anne Fine you'll know that she's skilled at writing from the child's point of view and they made me look at the step relationship in a different way.
The best thing about them being children's books is my DSD (then aged about 10) could read them too.