Back when Mac Classic was the Operating system for Apple there was a good book that was almost required if you wanted to fix Macs called SE Repair and Upgrade Secrets. It was written by Larry Pina and it went into detail about how to repair and upgrade a Macintosh Classic and Macintosh SE personal computer, without spending more money than would be required to buy a new one.
It was the bible almost when it was first published by Peachpit Press in 1993, since then its been out of print. All Macs now since 2002 ship with Apple OSx which is based upon the Mach kernel. Certain parts of which are from FreeBSD's and NetBSD's implementation of Unix. These were incorporated into NeXtStep which serves as the core of Mac OS X. NeXTSTEP was the UNIX-based operating system developed by Steve Jobs' company NeXT after he left Apple in 1985. Apple tried to create a "next-generation" OS through the Taligent, Copland and Gershwin projects. Where Apple failed though, Steve Jobs succeded as NeXtStep was the graphical, object-oriented, and UNIX-based operating system
Eventually, NeXT's OS was selected to be the basis for Apple's next OS, and Apple purchased NeXT outright. Steve Jobs returned to Apple as interim CEO, and later became CEO, shepherding the transformation of the programmer-friendly OPENSTEP into a system that would be adopted by Apple's primary market of home users and creative professionals. The project was first known as Rhapsody and was later renamed to Mac OS X.
All Macs have run on PowerPC-based CPU's but In 2006, the first Intel Macs had a specialized version of 10.4 "Tiger". In 2007, 10.5 "Leopard" was the first to run on both PowerPC and Intel Macs with the use of Universal Binaries. 10.6 "Snow Leopard" was the first version of OS X to drop support for PowerPC Macs.
Under the hood there is one thing that is undeniable though, OSx is Unix so much of your documentation is there in man pages, lots of documentation in fact. Enough that you may never need to buy a book again.