Iāve read both Karate: Technique and Spirit, and The Human Face of Karate. Both are quite good IMO. Both are technically required, per the syllabus, but I havenāt heard of anyone taking a test on them by any means. My CI has a copy or two if someone wants to borrow them.
I bought Technique and Spirit during my first stint in karate (non-Seido) as I thought it was a great book. My interest was mainly his philosophy as it relates to the study of karate. Thereās technique stuff in there too, which the main allure to Seido students is most likely terminology. Thereās a history section which is a bit suspect IMO, but itās ok. It isnāt intentional nor slanted to benefit him, Seido, nor anyone else.
The Human Face is his autobiography. Quite an interesting read. The earliest parts are about his growing up. It focuses heavily on his karate - his pre-Kyokushin days, his days with Oyama, being sent to the US, his departure from Kyokushin and the aftermath of it, and why heās taken Seido in its current direction.
Thereās some quite interesting stuff about his Kyokushin days thatās not common knowledge - he introduced the belt color system, the seniors were pretty opposed to it, yet Oyama allowed it; the struggles in running Camp Zama, which was a US military base, etc.
Technique and Spirit can be found pretty cheap new and especially used on Amazon here. Human Face isnāt very common; I ordered it directly through Honbu for I think $20 or $25? Both are worth their new price IMO. I bought Technique and Spirit brand new in a bookstore before Amazon (and the internet, for all intents and purposes) was a thing. I didnāt regret spending the money. Again, this was about 20 years before my Seido days, so no real bias on my part.