The problem with your question is that if you ask five different people you will probably get 8 different answers.

I would second Iron Coffins. Although it is full of "liberities" ,it is an excellent book to read as your first book on the U-boats. It is very readable and gives you just enough background to give you a good basic understanding.

I am sure many of us started out with Iron Coffins. I know I did.

Then, once you have read that book, you will have a much better understanding of where you want your personal research to go concerning Uboats.

Like many historical topics, there is no one best book. It all depends on what aspect of the UBoat history appeals to you.

Good luck with that. Let us know some specifics about what interests you and we will be glad to point you to some excellent books