(updated - this offer appears to still be valid as on 2 Dec 09)
The Lonely Planet site allows you to purchase digital downloads (PDFs) of popular travel guides by Chapters, so that you don't have to buy the entire guidebook if you're only planning to visit a few places within a region (eg. Europe, China, etc.).
The PDFs are also more portable if you have a device (eg. mobile phone, netbook) that can read PDFs. Alternatively, you can always print some of the pages out (eg. Maps) - saves carrying a v thick book with you.
If you purchase 3 or 4 digital chapters, you can save 25%.
If you purhcase 5 or more digital chapters, you can save 45%.
However, I did note that when I purchased 10 or more chapters from a particular guidebook, the savings was 80%!
The digital chapters I purchased were all from the "Europe on a Shoestring - 6th Edition" guidebook:
http://shop.lonelyplanet.com/Primary/Product/Pick_and_Mix_Ch…
What would've been AUD$62 in total came down to AUD$12.40.
(note that the 80% discount was from the same guidebook - i don't think the same 80% discount would apply if you mix-and-match 10 or more chapters from various guidebooks - might be a different percentage instead).
No idea when this offer will expire - doesn't appear to be specified on the site.
Credit to Lonely Planet for selling their chapters in PDF format, and not some awful DRM-encrusted format.
This can be very convenient.