Access to information on the Apple2’s is as good and maybe better thirty-odd years later. Every book that I recall (and many that I don’t) are found as PDF scans or even OCR/hand-edited versions in useful formats (epub, mobi, text, pdf). Here’s my link dump…

Books

The community has collected many (hint, search archive.org), linking to some of the best.

Apple Manuals:

• Apple II Reference Manual
• Apple II Monitors Peeled
• Apple IIe Reference Manual
• Apple IIe Technical Reference Manual

Notable books from the era:

• Little, Inside the Apple IIe
• Gayler, Apple II Circuit Description
• Lechner & Worth, Beneath Apple DOS
• Hyde, Using 6502 Assembly Language
• Zaks, Programming the 6502
• Leventhal, 6502 Assembly Language Programming
• Leventhal, 6502 Assembly Language Subroutines
• DeJong, Programming & Interfacing the 6502, with Experiments
• Brenner, Apple II Plus/IIe Troubleshooting and Repair
• Lancaster, Assembly Cookbook for the Apple II/IIE
• Inman & Inman, Apple Machine Language

Handy references:

• Nibble Magazine Assembler Cross Reference

Updates and reprints are available for some:

• Dougherty, The Apple II Monitor Peeled
• Luebbert, Martens & Wiser, What’s Where in the Apple Enhanced Edition
• Wozniak, Martens & Wiser, The Wozpak Special Edition
• Wagner, Assembly Lines- The Complete Book