Computer books get outdated so fast, but I still buy them once in a while even though I have an iPad just for reading ebooks. It's just awesome to have (almost) my entire library in ebook format and always have them with me, but they're somewhat poor as a reference tool if you need a whole series of instructions (such as doing an installation) and need to keep going back and forth between a number of pages or even chapters, and searching is really slow on the iPad. I've yet to try PDF reading on the iPad 2 (since I don't have one), but the faster processor could help with both issues.
Anyway, my most recent book purchase is Windows Server 2008 R2 Unleashed even though I already bought the ebook earlier. I've always bought Microsoft's own books when it comes to Windows servers and I wanted to get Windows Server 2008 Inside Out but for some reason it wasn't updated for 2008 R2. I know that R2 doesn't have that many new features, but it's nice to have the latest edition and to prevent a new edition from being released the instant I buy the current edition. The book is 1600+ pages and has thin pages similar to the pages you normally find in the Bible, so physically the book is not that thick, but it's really heavy and I get a free workout every time I pick it up.
It's not always a good idea to buy the ebook then the paper book. O'Reilly has an offer where if you already have the paper book you can register it then "upgrade" the paper book to an ebook for only $4.99. Of course, nothing stops you from registering a book that you don't actually own, but hey, honor counts when it comes to ebooks.
Before ebooks and the term n00b were invented, I uesd to buy lots of books, and I kept almost all of them. Running Microsoft Windows NT Server 4.0 was the one that got me started in all this.