If I form a sentence like:
I could read the book if I hadn't wasted my time.
Would it be considered a past perfect sentence with the part I could read the book being in simple past for context in the sentence or does it only show a possibility?
Your sentence is grammatical as written but to make it grammatical the meaning may or may not need to be changed from your original intent.
Suppose you had been given several jobs to get done earlier in the day but instead of doing them you wasted your time. Now, later, you wish to read a book but because you had wasted your time earlier you must spend your time finishing your assigned chores instead of being able to read your book. So you say,
I could read my book [now] if I hadn't wasted my time [earlier].
No, it has to be:
I could have read the book if I hadn’t wasted my time.
Because you need both halves of an if–then construct to show up in the same perfectness, so to speak.