Here are some books that can help you work on your sight-reading chops...
A Modern Method for Guitar - Volumes 1, 2, 3 Complete - This is good one for beginners at reading. But it progresses pretty quick - stacked triads on page five!
Advanced Reading Studies for Guitar: Guitar Technique (Advanced Reading: Guitar) - more of the same
Melodic Rhythms for Guitar - This is a great book as it teaches you new rhythmic figures on one pitch and then uses them in realistic melodies.
Contemporary Jazz/Rock Rhythms for Treble Clef Instruments - Heres another good one.
For Guitar Players Only - By Tommy Tedesco the most recorded guitarist in history. I had this book. The exercises are a little random, which can make for good sight reading in it's unpredictability. Worth it for the stories. All true. And for the real session charts from the 70's and 80's in the back.
Also check out trumpet and clarinet study books. Flute is good for reading ledger lines. You might be able to find them at used book stores or thrift stores. I've never payed more than a dollar for one.
Practice your reading with another guitarist. Maybe someone better than you. Like playing tennis with a better player it forces you to rise to the occasion.
Here's the thing, if you are not likely to use it you are not likely to retain it. See if you can fine some reading bands, often swing bands or big bands that don't perform so much as practice. The Wire Choir here in LA is an example of one such opportunity. Also start writing music out. Write practice pieces for yourself that emphasize your weaknesses.
I worked it when I was in HS and college because I wanted to move to LA to be a session guitarist. Which I now am. Still surprised how rarely I need it. Read more cocktail napkins than bona-fide charts.