Actually, the verb DOES in fact agree with the subject, which is “series.” So, “series was” is grammatically correct. The problem is the subject complement “the last books that I read.” They should have said “the last SERIES of books that I read.” Now it all agrees.
Sadly, I think this may have been mine! Haha. I know I have these books under my Favorite Books section, but I honestly do not remember if I actually said that on my profile. I probably did. Who knows? However, I do like these books and I’m barely religious. If at all. Just thought they were a good read.