If I knew you had known that he read the book, I would ~~.
2. If I knew you had known that he had read the book, I would .
3. If I knew you knew that he read the book, I would ~~.
4. If I knew you knew that he had read the book, I would ~~.
5. If I knew you know that he read the book, I would ~~.
6. If I knew you know that he had read the book, I would ~~.
It's getting the logic of the time sequences sorted in your mind.
Take : If I knew you had known that he had read the book, I would (have)
If I knew : the 'knowing' part is sometime in the past. Let's say yesterday.
you had known : When we spoke yesterday, I found out that you already knew about (whatever the hell we are talking about 'knowing). You must have heard about it even earlier, say, the week before, so even further back in time. It is a completed action in the past.
So - so far, we have : past tense + something that happened even 'past-er' in time. We can indicate this sequence in time because past perfect puts something even further back in time, so we have past tense +past perfect tense.
that he had read the book,: again past perfect because the book must have been read by somebody earlier than any of this other taking place, the 'knowing 'and the knowing that you know' .