There are many books available on Reiki and associated topics and it can be difficult to choose. The following are my favourites:

A Course in Miracles 
A little light on Ascension - by Diana Cooper
The Celestine Prophesy 
Anatomy of the Spirit - by Caroline Myss
The Prophet by - Kahlil Gibran
After the Ecstacy, the Laundry - by Jack Kornfield
Emissary of Light - by James Twyman

And books by these authors:
Louise Hay books,  Elizabeth Kubler Ross, Robert Holden and all Reiki books!

There are many sources from which to purchase Reiki books including local bookstores and specialist mail order suppliers i.e. the Tao of Books (0845 3456 222) and the excellent Cygnus book club (01550 777701).

I have included the following link to Amazon Books in the hope that you may find it useful!