Practicing single lines is good but you also should learn to read the notation of chords on sheet music. Not as simple by a long shot.
This website is really a good one for such things.
Ricci Adams' Musictheory.net (Exercises/Chord Identification)
If you practice a bit every day you will make real progress.
Other excellent books for sight reading:
William Leavitt - Modern Method for Guitar Volume I
Tom Dempsey - Sight Reading For The Contemporary Guitarist