Exercises in books are very boring for a beginner , and of course you get frustrated when your friends ask you to play something cool and you don't know anything but your boring exercise you learned from the books you have.
But this doesn't mean to stop learning exercises cuz you'll realy benifit from these exercises but you still don't know how.
Exercises are usualy small riffs or chops taken from real songs for training on each one individualy, you'll find most of the exercises you've learned in songs you're going to play in future.
Anyway, since you already know some chords you should start learning some easy songs based on these chords.
Try learning some beatles, nirvana or other similar stuff you might find easy for you.
You'll find tabs for these all over the net.