Definitely buy them if you've not read them already, the last full book is out in a week so I suspect there will be offers on the previous books on Kindle etc, and there is a book collecting all the short stories (some of which explain bits that happened between or before the main books) coming out in March.
If you're more an audio person the audio books are quite well done as well.
Definitely gonna be keeping my eyes open for some deals! I've never listened to an audio book so could potentially give that a go too.