I've read of the book many times before. I remember flipping through the preview in Amazon once or twice to see if it looked any good. It looked like a rigorous study of every permutation of one-bar rhythms. (Correct.) It looked boring. (Wrong.) It looked like it had nothing to do with sight reading. (Wrong.)
When I began taking sight reading seriously, about two years ago, I thought all I needed was a wealth of material to practice. I bought The Real Book, started slow, and gradually progressed (slowly). Since I've been taking jazz lessons this year, my teacher, Travis, has repeatedly stressed "rhythmic recognition." It's generally more important that I play the correct rhythms than the correct notes, so I need to see rhythmic phrases in the same way we read words. Just as we see and know a word instantly without picking out individual letters, I must do the same with rhythms.
Travis's and Connor's advice came at the same time, so I decided to check out Leavitt's book. Over the last few weeks alone, it's done more for my sight reading than anything else I've tried. It starts by looking at every possible way to play seven notes in a bar, with nothing faster than eighth notes. Then you get a few etudes utilizing all those patterns. Then every permutation of six notes, etudes, etc. It has a highly effective structure of providing small chunks of new vocabulary with practical applications to study. It's about 70 pages long, completely full of music, very little text, and I'm halfway through it already.
I also bought Reading Studies for Guitar, Advanced Reading Studies for Guitar, and Classical Studies for Pick-Style Guitar, all by Leavitt, in the same order. They look promising, but I haven't cracked them open yet.