The same thing happened to me a few days ago; immediately after I purchased a new book. I don't know if it is related or coincidental but it is certainly frustrating as I read every day.
I've tried every possible thing I could think of and other suggestions from the internet. Nothing works.
Here's some of what I've tried to no avail:
1) Hard restart of my iPhone (e.g. pressing Home button and Off button at the same time)
2) Turn iPhone on and off
3) Remove all my books and re-download one or two
4) Make changes in Settings as suggested on the internet
5) Check the Apple website to see if they were doing maintenance on the iBooks app
6) Checked that my iOS version was up-to-date
7) Checked that my iBooks version was up-to-date
8) Ensure I had not accidentally turned off iBooks access to cellular data
9) Delete and re-install iBooks app
10) Check from time to time if the issue is resolved without me doing anything
11) Submit the issue to Apple (submitted on the 25th at 5 a.m. and waiting for a response)
On occasion after a hard restart, I can see the content of one book but never for my most recent purchase. I'm at a loss trying to figure out what is happening and how to resolve the issue.