I also got the pop up at goal 1 of a squires book as a reward and nothing was in my inventory. I got another pop up after "A Plushy Knight's Tale" dropped from the library with a squire book as a reward, but again that also wasn't in my inventory. So I think the "reward" book and the Knight's Tale are different items? I'm wondering if the goal reward book is really unlocking something we don't quite realize yet? Your guess is as good as mine.