These books were printed on cheap(ish) paper stock. They yellow, fox, and the glue bindings degrade. Finding a copy where the spine is intact and the pages are "off-white" rather than "brown" is a miracle.
Most of Rika Nishimura’s early 90s works, including the "Music Alone" series, are now out of print and typically found through specialized collectors or vintage shops like Retirement: Rika Nishimura Photo Book Music Alone Junior A