Think of season five as "how we got here", and season six as "what we do about it." That is, if it stays true to the books which it certainly appears to be true. It's more than a setup season, because of the growth and progression of characters, but probably won't leave folks satisfied, IMO.
As far as reading the books, I recommend them highly but they really don't tell a different story than the show. Sure, you get some individual book characters who are kinda smooshed into one show character, but essential the story is identical. I don't see that as a bad thing, but if your expectation is get a lot more detail, and exposition from the books (say like with Song of Ice and Fire compared to GoT) you might be disappointed. The one exception, for me anyway, is the novella The Churn. If you want to know bunch more about Amos/Timmy then you should get it right away. It's inexpensive, a short read, and super good.