These are the most interesting books, they can all be found on Asimov repository. Reading these books, you can notice how much times have changed if you compare them to the information you can get, even as a developer, from hardware constructors. These books gives you an amazing amount of information : electronic schemas, ROM listings, specifications...
This is the bible. Almost every thing is explained. I have the French version. The English original version can be found there :
ftp://ftp.apple.asimov.net/pub/apple_II/documentation/Apple IIe Technical Reference Manual.pdf
This is an older bible. It is also known as the red book. Fortunately I got one from ebay. A PDF version can be found there :
ftp://ftp.apple.asimov.net/pub/apple_II/documentation/Apple_II_Redbook.pdf
I read the chapter 8, about the Disk II Controller, as official Apple manual does not provide any information on it. I haven't read the rest of this book (I have no paper version) but it looks very interesting and goes deep in Apple II description. A PDF version can be found there :
ftp://ftp.apple.asimov.net/pub/apple_II/documentation/understanding the apple ii.pdf