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.)