For some reason I was cleaning up my books in Play Books and accidentally removed a title I wanted to have. When I went to pick up a new copy in the Play store, the lost title had a Read button and allowed me to read it on the web. So I knew that it was still somewhere waiting for me.
I was pretty stumped for a while about how to fix the problem, then thought to go through my Play Purchases in my account (in the Play store, look for the cog in the upper right hand side). And there it was, the lost title and the three dots to click on and choose Show in Library.
I still don’t have the Play Books update, but will review it when I do.