is the rest of the book(s) a bit of a slog as well?
The first three books are excellent. Book three, A Storm of Swords, is the best in the series so far in my opinion. The TV show kinda keeps pace with the books, the first season is a pretty faithful recreation of the first book, the season season is the second book (albeit with a few changes) and the third season is probably about half of the third book with some stuff from book two that they left out of season two and some stuff that (I presume) happens in books four or five that I've not read yet.
If you have been watching the show then there is still a ton of stuff to happen in book three which will .probably make season four. I will say if you want to start reading start on book one rather than attempting to jump in where you think the TV show is.
As for the specifics about book four, well I'm a little over half way through (400 of 700 pages) and the book has been all set up no pay off so far. It's alright when you have a lot invested in the characters and some of the storylines are still very good but there are a couple which really do feel like a slog.