In re: hypnosis in general the books I recommend are: "TRANCE-formations" by Bandler and Grinder, and "Monsters and Magical Sticks: There's No Such Thing as Hypnosis?" by Heller.
For specifically improving "cognitive enhancement" the easiest, safest, and most far-reaching technique involves modifying your self-image. You can reprogram yourself to believe that you're smarter and your mind will modify your behaviour to meet your belief. For better or worse one's intelligence level is generally tied into one's self-image, and the only barrier to being smarter is your natural reluctance to become a different person.
For self-image engineering the best books I've found are "Psycho Cybernetics" by Maltz and "With Winning in Mind" by Bassham.
https://www.goodreads.com/book/show/447454.Monsters_and_Magi...
https://www.goodreads.com/book/show/956297.Trance_Formations
https://www.goodreads.com/book/show/155981.Psycho_Cybernetic...
https://www.goodreads.com/book/show/208926.With_Winning_in_M...
(But really, read "Prometheus Rising" by R. A. Wilson and do the exercises.)
Usually around October, as the temperature falls and the sun sets earlier, I delve more into the esoteric. RAW has always been a go-to and his lessons are eye opening. When I finally manifest my quarter I was ecstatic :)
The Master Key system is an interesting read too https://www.amazon.com/Master-Key-System-Charles-Haanel-eboo...