Life is better today.
It´s not the first time that a grammar kink comes up in a textbook, but you can excuse the publisher if it is good in every other aspect and you don´t see many such mistakes.
Anyway, it raises someone´s eyebrows because it is a basic one - even I could spot it, mind you! Plus It may leave you not to lower your guard so you could expect more... I hope it isn´t plentiful, though.
On the plus side, it can be a good thing that you felt that feeling of "did I just read that?" it means that you are learning!