A friend gave me two books. One was a love expert. One was the psychology of love. He did not know if it would be useful or not.

Answer 1

Love Expert and Love Psychology are both useful books. You can refer to the following suggestions:
A love expert might help him better understand female psychology and communication skills so as to better grasp the rhythm and heat of love.
Love psychology would allow one to understand the nature and laws of love, better understand their emotional needs, and better express and satisfy their emotional needs in love.
Of course, the specific effect depended on the content of the book and the individual's situation. No matter which book you choose, you should read it carefully and make appropriate reflections and adjustments according to your own needs.