I didn't see anything already posted so apologies if I missed it, but over the weekend, as I was perusing my Comixology app, I noticed that DC is now offering digital collections. Before, they would group the individual issues together so you could purchase a complete story, but didn't offer the digital version of a TPB like Marvel and other publishers did.
That is no longer the case and you can now do the same for DC Comics.
But, before you jump in and start shopping, you should also know that DC started offering collections like this on the Kindle store before they did the same for Comixology and a lot of the Kindle Editions are lower priced than the Comixology version. If you're using a phone or tablet, then either way you have access to the app (and for free), so unless you have a preference for one or the other, make sure to check both stores before buying. I've actually tried out the Kindle Edition of a few, including Books of Magic, and Fables Vol. 1, and if the panels are small you can double-tap them and each panel will pop out a little bigger and easier to read, so the experience is pretty similar to Comixology.
I recently bought the digital collection of Superman: Birthright, and the Kindle Edition was $9.99, a better deal than the $12.99 on Comixology. Checking some of the other offerings, the Kindle Edition seems to be less expensive across the board.