Paperback and hardback books are available online from retailers, including:

        Barnes&Noble (USA)      barnesandnoble.com
        Quail Ridge Books (USA)      quailridgebooks.com
        Powell's (USA)      powells.com
        Blackwell's (UK)      blackwells.co.uk
        Foyles (UK)      foyles.co.uk
        Waterstones (UK)      waterstones.com
        Bokus (Sweden)      bokus.com
        Adlibris (Sweden)      adlibris.com/se
        Fishpond (Australia)      fishpond.com.au/Books
        Indigo (Canada)      indigo.ca
        Fishpond (New Zealand)      fishpond.co.nz/Books
        Exclusive Books (South Africa)      exclusivebooks.co.za
                 and on Amazon.

e-books direct to readers are now available.
Worldwide sales via VitalSource.com, who also sell e-books in bulk.
Some American readers may choose to use RedShelf.com.
Canadian institutions may contact Follet books for their bookstores.
Scandinavian readers can buy from bokus.com and adlibris.com/se.
The e-books are considerably cheaper than the paperback editions.

University library e-books via EBSCO and via Publizon.