This won't interest most, but Network Theory is now publishing books of the official documentation for PostgreSQL (an excellent open source and free database system). More information at http://www.network-theory.co.uk/postgresql/vol1/.
Remember 5% Quidco, which for this particular offer is showing as £1.18 on my account, so you could say it's £22.50.
Note: These books are print on demand, so you have a wait a short while for them to arrive.
This is the cheapest I could find it. For volumes 2 and 3, I've found it's better to buy them both from Amazon.co.uk with Super Saver Delivery. (vol 2 [programming guide] is £13.16 and vol 3 [server administration] is £9.20, so they total £22.36). The next cheapest I could find was Amazon.co.uk for £28.99.
If you wish to search by ISBN, they are as follows:
Vol 1: 0954612027
Vol 2: 0954612035
Vol 3: 0954612043