G-C Rota has this to say about the Schaum's Outline Series:
Most mathematicians who teach mathematics fail. They bask in the illusion that the majority of their students should become mathematicians, or their teaching is wasted; or in the illusion of immediate, effortless understanding, the illusion that it suffices to present the "facts" and let students figure out the "sense" of the mathematics, a sense that substantially differs from the "meaning." Authors of Schaum's Outlines have learned to avoid these illusions. Anyone who is about to teach the undergraduate mathematics
curriculum should come down to earth by looking through The Schaum's Outlines before burdening the class with those well printed, many-colored, highly advertised hardcover volumes that are pathetically passed off as textbooks.
I may only add that Balakrishnan's is the clearest book I ever read. I do not mind admitting that it contained enough material to answer my combinatorial needs so far. Some material, new to me, has inspired several pages at the site.