For a short little book, it sure packs a punch. Most of Lewis' books are very accessible, this one may push the reader a little more, at least in part because of the number of references he makes to contemporary people and things. However, it is well worth any effort required. His critique of modern education not only remains valid, it is more relevant than ever, making Lewis seem a seer in hindsight.
He attacks the way that educators "debunk" even that which cannot be debunked, that is to say, first principles, assumptions, values, and the like. He works out for the reader how there is no way to arrive at any set of values "objectively" or "scientifically," because ultimately all systems of value depend on unprovable assumptions. He makes the assertion that those who accept some system of value, largely transmitted through history, though subject to change and reform from within, are essentially part of one stream of humanity, those who belong to the Tao (as Lewis chooses to refer to it). This may be the weakest part of his argument, as it could be argued there are more than one Tao that people could belong to, each being uncontroversially (to adherents) derived from first principles since antiquity. Lewis does throw in a few quotes at the end of how various human value systems have much basic about them in common, but this hardly amounts to an incontrovertible mountain of evidence.
The other alternative to accepting first principles as the foundation of a value system is to deny all value. In this, Lewis sees a more clear and present danger, as this second approach at least has internal logic and does not necessarily disprove itself, as does the "debunking" school (beware, fact-checkers!). Those operating intentionally without reference to assumed values are liable to behave according to their whims, their pleasures, etc. When this viewpoint is mixed with the science and material progress much in evidence in the human race, there rises a terrible potential for some group of humans to decide to program in a set of their own, arbitrary values in many (or perhaps all) future humans, through eugenics, education, propaganda, etc. After all, "what we call Man's power over Nature turns out to be a power exercised by some men over other men with Nature as its instrument." (p 26)
As a critique, it is outstanding. As Lewis often does with the razor of his mind, he slices a number of contrasting viewpoints to shreds and leaves one wondering if there could be any defense to such a withering review. As a proponent for anything new, Lewis is provocative but hardly comprehensive. He suggests imagining "a new Natural Philosophy, continually conscious that the 'natural object' produced by analysis and abstraction is not reality but only a view, and always correcting the abstraction." (p 35) So it turns out the "what next" part is up to you, dear reader, not provided in blueprint by Lewis.
This is perhaps one of Lewis' most secular works; it most certainly is not an apology for Christianity or an explanation of any of its tenets, per se. Obviously, Lewis' Christian belief and practice no doubt mold his mind to view all assumed values as belonging to one large stream (the Tao he references), even with their evident differences. It is much more a work of philosophy, of ethics and morals, than it is anything theological or religious. His references to religion are more in the line of purely academic, using texts from varying religions as sign posts to the values assumed and passed on for millennia.
The work is probably most famous for Lewis' phrase, "Men without Chests," by which he means humans without hearts. In Lewis' diagram, the mind is spiritual, the body animal, and it is the heart (or Chest, as he calls it) that unites them and makes people truly human. While this point is important enough to deserve its own chapter in this short work, it really is not the most significant part of reading this book. I would highlight the main benefit of reading it as such: this book should be given as a graduation gift to every graduating high school senior, so they can begin to understand what has been done to them for the prior 12 years and to help them to begin to recover from all of that. 
One quote bears repeating here, as it can seen from it how prescient Lewis was and just how current his work remains:

In a sort of ghastly simplicity, we remove the organ and demand the function. We make men without chests and expect of them virtue and expertise. We laugh at honor and are shocked to find traitors in our midst. We castrate and bid the geldings be fruitful. (p 14)

I would categorize this as a must-read. It's true that it is written at a higher level and would challenge at least some readers. But the real challenge is the substance, the argument. If you agree with it or not, Lewis' strength of reason and rhetoric make this a worthwhile adversary with which to contend, or perhaps a guiding light to illuminate a path to truth and purpose. That part lies with you...