The Mahatma Letters to A.P. Sinnett is one of the books that is central
to theosophical literature. I read, and reread it a lot, in the early
years of my being involved with the Theosophical Society and it's one of
the books I based my first online project on 'The Esoteric Studies
Guide'.

Joy Mills is famous in the Theosophical Society Adyar for her in depth
study of these letters. She's been leading Mahatma Letters study groups
for decades. As she's in her eighties, I think it's very lucky she
decided to gather all that wisdom and knowledge into a book, so when she
passes, her insights and encyclopaedic knowledge don't pass with her.

See my full review of her book 'Reflections on an Ageless Wisdom: commentary on the Mahatma Letters here: http://www.squidoo.com/ageless-wisdom