The Lonely Planet series are great and I was look for some new books as inspiration for my next holiday. Amazon is the cheapest I have seen the newest editions for although you can sometimes get lucky on greenmetropolis.com or charity bookshops.
Guide books available for many, many places, below are an example of just 3.
Australia (14th ed, Nov 2007) - £8.85 (rrp £16.99)
New Zealand (14th ed, Nov 2008) - £8.06 (rrp £15.99)
Thailand (12th ed, Aug 2007) - £8.07 (rrp £15.99)
This is probably due to the fact that these have new editions come out every couple of years so they have to reduce the prices before new editions are released. There are new editions of Thailand and Australia due out later this year but for anyone going on holiday or planning a holiday soon then I highly recommend these.