Start with high-confidence massage & reflexology books, then narrow by subgenre or adjacent topics.
Better Stretching
Joe Yoon
Healing Touch for Cats
Dr. Michael W. Fox
Chi Self-massage
Mantak Chia
The Trigger Point Therapy Workbook
Clair Davies NCTMB, Amber Davies CMTPT LMT
Complete Guide to Foam Rolling
Kyle Stull