It’s important to Write about what you read, but instead of just writing about the specific book you’re reading, you can (and should) write your notes such that your reading observations accumulate over time as they interact with each other and with your own ideas (see Evergreen note-writing helps insight accumulate, Knowledge work should accrete).
This is also why we write Evergreen notes: so that if we encounter a book which discusses a concept we’ve already written about, we’re pushed to integrate new ideas with our prior conception. Certainly, we normally do this when we read, but we’re limited to our faulty memory of other works which might be related. The externalized note-taking system substantially removes this limitation.
This is why part of why Evergreen notes should be concept-oriented: so that the structure of our notes pushes us to notice the relationships between the ideas in different texts—and in our own work (see Evergreen notes should be densely linked, Notes should surprise you).
The notes you write will also produce the foundations of new manuscripts (Executable strategy for writing).
This is one reason for Evergreen note-writing as fundamental unit of knowledge work.