Joel recommended starting with Living the Infinite Way and Practicing the Presence, followed by The Art of Meditation and The Art of Spiritual Healing. You can also browse all of the books, and if you find a title that resonates with you, start there.
In A Message for the Ages, chapter 11, “Spiritual Unfoldment through The Infinite Way," Joel says:
“Based on my own experience, I recommend that students begin their study with Living the Infinite Way, spend a couple of days on the Introduction, and then go on into the book. Following that, the student should devote two or three days to the Introduction to Practicing the Presence. Why those two or three days on the Introduction? Because it provides a foundation, explaining to the student the object of the book, what he is to attain, why he is to attain it, and giving him a reason for all that follows.
“To read books of this nature without knowing why or what the expected result should be is folly. All that happens when a student merely reads a book without understanding of its purpose is that he has read another book which was very sweet and pleasant to read, but nothing has been accomplished. After a few readings he should go back to the Introduction and ask himself, ‘Did I get out of the book what its purpose is?’ If not, he must go back again, because these books are not just to be read: they are books that are to be worn out until new copies are needed.
“Then I turn students to The Art of Meditation and The Art of Spiritual Healing and from there on, into any of the books in any order. I always bring to light that the books of The Infinite Way Letters, however, are as important as any books we have, if not more so. [Note: The 1954 – 1959 Infinite Way Letters are collected in the six-volume series titled The Heart of Mysticism.] Here in every chapter the student is introduced to certain principles and their application, so that he can always go to those books and find working tools ... No student is going to advance in The Infinite Way without an understanding of the foundational principles. On the other hand, no advanced student is going to succeed unless he keeps going back to the basic principles.”