I haven't read the books; but I know who A is in them... However, from what I've seen so far the series isn't really following the books. There are a couple of characters that have lived longer in the series than in the books
I've read the first 4 books so I knew about Mona but then I started watching the show and since the books and show go off track a bit I decided to just watch.
I'm only watching the series as well, but I would love to read the books, from what I've researched so far they sound pretty interesting