On Justin Sandercoe's site he recommends a flute book for guitarists, this caught my eye:
Why a flute book? because most sight reading books for guitar are written for guitarists and are either in position or are mind numbing boring random notes and scales. This book is full of short beautiful melodies by the greatest composers so the reading is actually fun! As well as that the melodies are real, not exercises so you get "real" experience of reading real melodies. They are also not in position so you get used to choosing a position to read in. There are many time signatures covered too - so you can develop that skill.
I've never heard of the phrase "in position" or "not in position", can someone explain what it means exactly?