The nondual traditions often downplay books as instructions for spiritual growth, preferring life, practice and verbal dialog with awakened ones. Never the less, I have found the following few books helpful and enjoyable in deepening  understanding of the subject.

Suzanne Segal

An American women permanently loses her sense of personal self at a bus stop in France. Very good description of what it is like to loose a separate identity without the guidance of a teacher.  I bought this book because I liked the cover.  Then  the real life description of the 'experience' of waking up  inspired the creation of this website

H.W.L. Poonja

Dialogs with Sri Poonjaji

Direct observations form the nondual in a conversation format. Poonja is the same person as Papaji refereed to in many of the tales of awakening on this site. The format is Question and Answer, so you get a feeling of what satsang with Poonja would have been like Excellent.

Thich Nhat Hanh

The story of the Buddha stripped of metaphysical add ons. Very enjoyable to read.

Maharaj Nisargadatta 

Excellent first person account of living in the awakened state.  Dialog format. Clicking on the book will take you to Amazon where 21 out of 21 people that reviewed it gave this book 5 stars out of 5 stars. 540 pages.

Dzogchen & Padmasambhava (no link)

Sogyal Rinpoche

An overview of Dzogchen. Published by Rigpa Fellowship in 1989, this is a 103 page readable overview of Dzogchen. Believed out of print, but you might see this in a used book store.

Dzogchen, The Self Perfected State (no link)

Namkhai Norbu

If you really understand the title, I think you understand the nondual path.

A small 90 page book that provides a very good overview of Dzogchen. Much more to the point and readable then many Tibetan books on the subject.  Published in 1989 by Arkana, part of the Penguin Group

One Taste : The Journals of Ken Wilber