I've read a few chapters of the book till now and I think the diagrams/figures can be divided into two categories. 1) The figures which show heirarchy of classes/interfaces and 2) figures which explain execution flow or concepts in general. The 1st category of figures (along with the tables) act as a good reference and the index helps find them easily. Regarding the 2nd category if you check the index, some of the chapters like Streams and Multi-Threading have a plethora of diagrams which greatly help understand the runtime flow of programs and concepts like
Thread states. I really enjoyed reading the concurrency chapters, it is one of my faviorite topics.
For disclosure, I got a review copy of the book from publisher but these opinions are my own.