I only recommend
Plane Talk because I think it is an excellent resource for someone who is developing their soloing skills. I have nothing to gain personally in doing so, and only recommend it as it provides an excellent insight into approaching the guitar in a very musical way. There are other good resources available, but I do not believe they convey as much information as concisely as
PT does, nor provide such a solid base for future development.
PT is by no means exhaustive, and once someone has studied it and is ready to look at further options for soloing, there are other books I would refer someone too.