Clients often ask for more information about TA, this book is a great foundation. It was on our recommended list for counsellors at Level 4.
If you can afford to do a TA 101 Workshop with UKATA its also well worth the investment.
The Power of Now by Eckhart Tolle
The Sidhe: Wisdom from the Celtic Otherworld by John Matthews
Zodiac Gateways by John Wadsworth