After watching the episode especially the confession, KK seemed overconfident. He is confident that he will NEVER be caught. May be that's the reason he never got the book back from Sinha house. He was confident as he said during Kartik's wedding that the poisons in the coal mines don't leave a trace for anyone to find out how the person died, that book won't have any trace of poison. Also it will also be a proof of his INNOCENCE that he left the book and did not get it back if he was the killer. It makes sense to leave it there.
People in the forum were right that he got the autopsy done to prove him innocent. He was sure that nothing would be found.
Everything was done peacefully. Nobody got a frisk of what really happened with Prabhakar.
One more thing is that Siddhant was not exactly sure of KK being the killer until he heard KK talking about the poisons on Kartik's wedding. And Sidhant remembered Kartik's version of Rao being KK's shield. He put 2 and 2 together and came to the conclusion that KK killed his dad. Everything falls in place so well.
Sorry for the long post.