I subscribe to Safari on oreilly.com, which lets you have 10 electronic books at one time for $9.95 / month. You can switch books out after you've had them for awhile. What's good about that is that after you've learned something, you can switch those books out for something new you're learning.
I found the $9.95 option hard to find when I signed up -- they really want you to do the $19.95 / month option. 10 books has been enough for me though.
Charlie
