Although the details of the book are scarce we have been informed that it's 176 pages and appears to be based on himself and his life.
Oshima began his career in 1997 at the age of 22 and won his first 11 bouts before getting his first title fight in 2000. Unfortunately that title bout saw Oshima come up short against Rick Yoshimura in a bout for the Japanese Lightweight title, losing a 10 round decision to Yoshimura.
Oshima bounced back from that loss with a string of 4 stoppage wins before getting his second title fight, a bout with Masakazu Satake at 140lbs for the OPBF title. Unfortunately Oshima was beaten again, losing by 12th round KO to Satake and then he suffer back to back loss, sliding to 15-4 (11) as a result.
Oshima would eventually run off a 7 fight unbeaten run, with 6 wins and a draw, before fighting Norio Kimura for the Japanese 140lbs title, suffering a 4th round loss in that bout. That loss resulted in Oshima suffering visual issues and subsequently retiring from the ring.
The book is available here http://seluba.shop-pro.jp/?pid=108436085 and costs 1,620 yen.