edit: the book in the link looks just as good.
Had to take a mostly equivalent class (Intro. to Abstract Math) for my Math degree as well, and the professor explained the same concepts clearer and everyone seemed to come out with a good understanding.
1. This is an awful thing to say.
2. The grandparent is probably referring to Rosen's Discrete Mathematics and Its Applications (http://highered.mcgraw-hill.com/sites/0073383090), not his number-theory book.
That is true, but JoshGlazebrook didn't say "Rosen's book is terrible", but rather "Will give this a read through to see if it explains better than Rosen". It is certainly a fair thing for him to struggle personally with the explanations in a textbook, no matter how good it is; and turning this constructive comment into occasion for a personal attack / judgement is, I think, unnecessary and unconstructive. By contrast, chris_wot's comment downthread (https://news.ycombinator.com/item?id=7247572), asking what is unsatisfactory, seems far more likely to generate productive discourse.
(For what it's worth, I am using Rosen this semester, and find it roughly middle of the road; I like it, but can understand that it might not be for everyone.)