Huge fan of the books and the show, and although I share some of the fears of Martin never finishing the series, I also think it's a totally unique experience to be caught up on both the show and the books as they race each other to finish.
Last season, several episodes deviated from the books' plot in ways that could hint at what storylines are the most important in the still to be written novels. Rumor has it that the writers of the show have been informed of the twists and turns all the way to the end of the series, and I believe they're crafting a companion piece that condenses Martin's story into something non-readers can get into.
***********SPOILER ALERT TV/BOOKS *************
I'll point specifically to the scene where the council of Others turns Craster's son into an Other. This scene has not appeared in any of the books, and confirms something readers had only speculated before: The Others are not a mindless race of evil that raises the dead and comes to bring the endless night. They obviously have a leader (The Night King of old Nan's stories perhaps?) and if there's a leader then there is a hierarchy and a society of some kind in place. None of this was any more than words (and words are wind, of course) until the show. I think the show will continue to be fed bits of important pieces of the story that will fire up the readers for the release of Winds of Winter.
I highly recommend the books as long as you have a strong stomach for fictional violence and abuse of women, animals, poor people, religious people, military captives.... well really just about everyone. There's also a million characters, and all kinds of foreshadowing and backstory buried in between the lines. The depth of the theories of what is truly happening in the song of ice and fire never cease to amaze me.
Can't wait for season 5! Or Winds of Winter.... which I can't imagine will be out before the end of season 5 😛