O'Reilly is running a special for members of User Groups, like IxDA. Here's the details:
Along with your 35% discount off print books, you can now get 45% off all ebooks you purchase direct from oreilly.com for a limited time.
When you buy an O'Reilly ebook you get lifetime access to the book, and whenever possible we make it available to you in four, DRM-free file formats--PDF, .epub, Kindle-compatible .mobi, and Android ebook--that you can use on the devices of your choice. Our ebook files are fully searchable, and you can cut-and-paste and print them. We also alert you when we've updated the files with corrections and additions.
Just use code DSUG when ordering online at www.oreilly.com/store
Read more about our ebook formats and the ways to use them here: http://oreilly.com/ebooks