You can get sale alerts for all the books on your Goodreads "want to read" list. Whenever one of them is discounted on Kindle, iBooks, Nook, Google Play, or Kobo, you'll get an email. (This only works for ebooks, not physical copies.)
On the Goodreads web interface, go to your account settings and click "Deals." Select "Deals from my Want to Read shelf." You can also get collections of deals according to author, genre, or popularity, but that can clutter your inbox.
With these settings, you'll just get alerts for the books you explicitly want to buy. I have 157 books on my reading list, and one goes on sale every two or three weeks. It's so convenient it feels like cheating.
The deals are extreme: I grabbed Terry Pratchett's Wintersmith for two bucks, A People's History of the United States for four, and A Wizard of Earthsea for three. Books this cheap might be bad for the industry! But at these coffee-sized prices, you can afford to buy twice as many.