I've taken to getting a lot of astro books as downloads from Google Play. (I imagine other e-book services like the iTunes store would be similar.) They have a pretty good range, and you can download them straight away, rather than trying to hunt down or order-in a copy. They're usually cheaper than hard copy as well.
E.g. "Turn Left at Orion" is $17.80 as a Google Play download, or $52 in hard-copy at Dymocks:
https://play.google.com/store/books/...d=PexKTfPy3voC
https://www.dymocks.com.au/book/turn.../#.VVqzyfm1A40
Google Play has a good range of astro books available for around $15 - $30, including the excellent "Patrick Moore's Practical Astronomy Series":
https://play.google.com/store/search...ronomy&c=books . These books can be quite hard to find in book stores, and are quite a bit more expensive when you can track one down.
On Google Play, you can download a free sample before committing to buy, to see if it is targeted at the right level for you, covers the subjects you're interested in, etc.
I still find a tree-book is easier to use than an e-book for reference use, or if it is full of detailed graphics etc, but the convenience of having your whole astro-library on your tablet wherever you are has to be considered as well. I buy more e-books than I used to buy tree-books (so even though they're cheaper, I'm probably spending around the same in an average year), but I'll still get an occasional hard-copy of a particularly good book (or if I see it in the bargain bins at my local book store).
My "Norton's Star Atlas" is a good example, where I have both formats - the general reference material is fine to browse as an e-book, but the star charts don't work well on a small screen, and need to be seen in full-size hard-copy to be appreciated properly.