If, after reading the book, you have the complete information you need and don't have to open it again, I think you should say: 'I'VE READ the book', because that past action, 'reading', led to your present 'knowing'. You have read it (past) and now you know it (present), as a present consequence of that past action: that 'past' is 'still with you'.
On the contrary, if after reading the book you haven't learned the book content or you don't remember it, I think you should say: 'I READ the book', because what you read haven't left a present consequence on your mind, because you don't remember the book content.
However, there is also another 'usage' of the present perfect, that is the description of a 'continuous' action, which started in the past and continued until the moment of speaking (or just before).
If you consider the action of reading that book in this way, and IF YOU'VE JUST FINISHED TO READ (a few minutes before), you could also say: I'VE BEEN READING, because you could also describe that action as an action you've been doing, that you began in the past and continued until the moment of speaking (or just before).
And if you've just read THE WHOLE BOOK, if you have completed the reading of the book, and you consider this second 'usage' of the present perfect, even if you haven't learned it, you could say: 'I'VE READ', because, with this second 'usage', the use of the 'present perfect simple' (I'VE READ) instead of the 'present perfect continuous' (I'VE BEEN READING) gives the idea of HAVING JUST COMPLETED an action (you've read the whole book).
With this second kind of usage of the present perfect, it doesn't matter if you have learned the book you've been reading, what matters is that you've completed the action of reading.
Just to recap, there are 2 kinds of usage of the present perfect:
1) To express the idea of a past, finished action that has left a present consequence, or that has a present significance, that is in a certain way 'still with us';