The Will: A person's innate potential for psionic ability and mastery.
The Way: The explicit and rigorous study and advance of psionic abilities.
This tome is one that should be regarded as reasonably essential for the Dark Sun Campaign as it probes the background of, and expands, the entire Psionic background that permeates all of Athas. But why should one have both The Complete Psionics Handbook -and- this one to provide the background? Well, the handbook is a generalist tome that provides a reasonable overview of the psionicist class, whilst the Will and the Way gives the essential Athasian twist to everything.
The introduction to the book provides an overview and how the Way emerged from Tarandas of Raam's school for the under-privelidged to have access to psionic training - thus ensuring that psionics would forever be the "great leveller" of Athasian society. Every playable race on Athas has psionics. Hence every PC also has them. But only a few are followers of the Way and therefore can consider themselves full psionicists - everyone else gains only wild (random) talents.
The first two chapters of the book delve in to some detail about how psionics interact with society, including how the various races regard psionics, and psionics around the Tyr region. The latter of these focuses on the various psionic schools around the major cities (sometimes with maps!) which provide a number of nice plot hooks for use by inventive GMs.
Various character kits feature in the third chapter. Of note, I particularly like the Sensei (of which one of my group had as a character kit back in the day). A blend of psionics with martial arts, the sensei is adept at close combat melee and dangerous, but provides the group with a notable combat spearhead coupled with strong psionic aptitude.
The third chapter on proficiencies is moderately passable, but the chapter after that on mental combat is where it starts to get much more interesting. It really expands what is presented in the complete psionics handbook with a very large chart on which psionic attack gets what die roll bonus against what psionic defence. The book introduces new concepts (harbingers and constructs) which better tie in with the background fluff presented in the Prism Pentad series. So, for example, if your attack is in the form of a bird of prey, and you put up a defence of a brick wall, the bird simply flies over the wall. Hence the bird would get a large bonus to the mental attack. This is what is essentially encapsulated in the large chart of attacks and defences.
The next two chapters delve in to the psionic disciplines (providing a very exhaustive listing of powers and their classifications and identifies what powers should be gained versus PC level) and research & meditation. These two techniques can open up new powers for the the psionicist and High Psionic powers (which are intended as the ultimate powers … which is somewhat curious given psionic enchantments that are possible when blended with preserving or defiling magic as used by avangions and dragons, but still very cool and very welcome additions).
The final proper chapter concerns itself with something that I think needs to be in every game of Dark Sun: psionic items. Whether these items have a personality of their own, or are the remnant intellect of a person now trapped and encased within an item, they should be more common on Athas than anywhere else in the Dungeons and Dragons universe -- particularly given how much time has supposedly elapsed since the appearance of psionics in the Dark Sun timeline. Indeed, the later book "Psionic Artifacts of Athas" takes this though process to its natural conclusion. As well, rules for PCs creating such items are presented.
In the appendix a slew of new psionic powers are presented. These include those that are more Athasian in implementation than the psionic handbook, coupled with more complete charts than found elsewhere (e.g. for wild talents).
Overall, this is a 4.9 out of 5 book for me. Sure you can run a Dark Sun campaign without it (which is the only reason it doesn't get 5 out of 5). But it'd be much richer with it! Twist my arm a bit, and its 5 out of 5. Which along with Earth, Air, Fire and Water, makes this book one of the two best products of the Dark Sun line beyond the campaign setting! Enjoy.