Just speaking as a (non-Alabama) faculty member, you have to be careful of Kindle-type books—the formatting is getting better, but surprisingly often diagrams and such don’t appear correctly on the virtual page. I’d look carefully at the reviews for a digital version before getting it—but if it’s formatted correctly (and paginated the same as the paper version!), then yes, go for it.
As for free scans of books online, in most cases (and unless an open-access textbook has been specifically chosen by the instructor) it’s a very real copyright violation, or an out of date edition.