Endgames are good. I always recommend Silman's Complete Endgame Course. It goes from easiest and most common material up to master level stuff that doesn't come up in games that often. So you can start out just reading the first chapter or two, learn some stuff that's good for beginners, then set it aside until you feel you want more endgame knowledge.
The other thing you need is tactics, besides just the checkmate puzzles you're doing. Those types of things are good, but in real games, tactics to win material like forks, pins, skewers, etc come up far more often. So those are where you should be focusing your attention. I'm really not sure what the best beginner books for that are. Maybe Sierawan's "Winning Chess Tactics" - I haven't read any of the books in that series, but I've heard they're all excellent.
Besides reading, are you playing regularly? And reviewing your games afterward to see what you could have done better? Those are key to improving. Head to freechess.org for a good, free playing site, if you're not already playing online somewhere.
And my one last piece of advice that I always give beginners - head to chesscafe.com, click the "archives" link, and look up Dan Heisman's Novice Nook column. Two of his early articles were called "A Generic Study Plan" and "The Four Homeworks". Start with those, then read the other 100+ monthly columns at your leisure.
--Fromper