THE Reference for scales and modes.
The first section can be a bit daunting, but I found it helped my understanding of scale and mode construction immensely.
Each scale also has a list of chords that the scale can fit with. I refer to it quite a bit for what scales go with what chords (or vice versa). It also lists modes for each and every scale.
I'd buy it again if I had to replace it. I say it is suitable for all skill levels, but I would imagine it would really be best used by intermediate players on up.