Juvenile Sci-Fi novel, at least 30 years old, trying to resurrect based on a fond memory.
An Earthman, hitman I believe, is captured by an alien. The hitman sneaks "tapes" for machine based learning (card playing, martial arts, operation of alien machines eg Mark V hovercar?), outsmarting the unsuspecting alien while becoming incredibly intelligent.
He then goes to the stars and some mayhem ensues. Ending involves him determining that his "helper" is a previously kindapped (Neanderthal) Earthling, finding out that he had unknowingly split his intelligence into two halves, and reuniting both halves of his intellect.
Any ideas? Google was not helpful despite numerous attempts. Thanks!