I thought I had a pretty good vocabulary until I started reading Hewett's Music: Healing the Rift! Here are some terms I found, previously unknown to me (most definitions courtesy of Merriam-Webster Online Dictionary):
the Einfall (p. 31) - German word for an "idea, notion, thought or incidence."
sui generis (p. 54) - Latin phrase meaning "of its own kind" or "unique."
solipsism (p. 54) - a theory holding that the self can know nothing but its own modifications and that the self is the only existent thing. Also, extreme egocentrism.
untrammeled (p. 54) - "trammeled" means 1) to catch or hold as if in a net, 2) to prevent or impede the free play of. "Untrammeled" is simply the opposite of these definitions, basically meaning unrestrained or free.
ineffability (p. 55) - "ineffable" = 1) incapable of being expressed in words; indescribable; unspeakable, 2) not to be uttered; taboo.
hypertrophy (p. 57) - 1) excessive development of an organ or part, 2) exaggerated growth or complexity.
inchoate (p. 61) - being only partly in existence or operation; imperfectly formed or formulated - formless, incoherent.
insouciantly (p. 62) - "insouciance" = lighthearted unconcern; non-chalance.
plenum (p. 63) - 1) the quality or state of being full, 2) a general assembly of all members, especially of a legislative body.
Ruthenes (p. 64) - a culturally-loaded term broadly referring to East Slavic peoples. A Latin term which refers to the Slavic Orthodox people who lived in the Grand Duchy of Lithuania.
neurasthenic (p. 68) - "neurasthenia" - a psychological disorder marked especially by easy fatigability and often by lack of motivation, feelings of inadequacy and psychosomatic symptoms - compare Chronic Fatigue Syndrome.
Hope this expands your vocabularies as it did mine!