There are the Berklee books " A Modern Method For Guitar" by William Leavitt which I've found to be pretty useful for sight reading. http://www.amazon.com/Modern-Method-Guitar-Volumes-Complete/dp/0876390114/ref=pd_sim_b_title_3
I think the musicians institute also brought out a series.
The important thing is to start slowly
, using a metronome, and trying not to stop when you make a mistake. Also don't play the same piece over and over - you'll just be memorising it! Rather rotate the music so that you don't play the same thing two days in a row.
You don't need to spend hours practising this - I've found that 15 mins a day is quite sufficient to start getting the hang of it. The thing is to do it regularly and methodically.