Nirmala has one of the easiest takes on nonduality, with many clear and simple examples, and a lucid writing style. One of my favorite books, and surprisingly little known.
Loch Kelly$18.95 $17.43
Jon Bernie and Loch Kelly both have backgrounds in psychotherapy and bodywork so are able to bridge the worlds of 'personal growth' and nonduality very nicely.
Joan Tollifson and Toni Packer are influential teachers who have a strong Zen feeling without being attached to any form called Zen or Buddhism. Adyashanti is another teacher who comes out of Zen but who doesn't use those forms any more. Gentle, quiet, beautifully written, and accessible.
Mooji is a charismatic teacher with many books and videos. I like his books the best-- they are pithy and unpretentious. Lots of ah-ha! moments.
Leo Hartong$14.95 $13.75
These two books represent what might be called 'radical nonduality', which questions even the need for spirituality or anything other than living our lives. Unlike the teachers more rooted in tradition, they reject the idea that anything needs to be done. You may love them or hate them. I have mixed feelings, but they will certainly make you think.
Sri Ramana Maharshi$17.95 $16.51
This is the deep end of the pool. Most people who read these authors have worn out copies of them, and even one page from any of them can keep you in contemplation for days. I'll be honest, I've never finished any of them, but pick them up, read a paragraph, then spend a couple of hours in a daze.