This is a very different book from McGrath's recent biography of Lewis (C. S. Lewis: A Life). It is a collection of scholarly essays on various aspects of Lewis's thought and life. Among the topics addressed are Lewis's views on myth, his so-called argument from desire, his apologetical method, his intellectual outlook in the 1920s (prior to his conversion), his Anglicanism, and whether he is properly regarded as a "theologian." Despite its somewhat misleading title, it is not a systematic attempt to reveal "the intellectual world" of Lewis. It is a loose and not-very-unified collection of "takes" on various aspects of Lewis's personality and thought that spun off from McGrath's research on Lewis's life, and is intended mostly for Lewis scholars and aficionados.
It is, nevertheless, interesting and significant. It is also bound to be controversial. McGrath argues, for example, that Lewis never intended the so-called "argument from desire" (roughly: all innate desires have a possible fulfillment; our desire for perfect and unending happiness is innate, so there must be a heaven where such a desire can be fulfilled) to be construed as an "argument" at all. To the extent that it is an argument it is an "abductive," suppositional argument of the form: We have desires that can't be satisfied in this world; if Christianity is true, we would expect to have such desires; so, there is some reason to think that Christianity is true. Such a reading is attractive to those (like McGrath) who think that Lewis's "argument," if construed as a deductive "proof," is clearly faulty. It is thus a charitable way of reading Lewis. Whether it is an accurate reading is another matter.
Another controversial view defended by McGrath is that Lewis wasn't a "rational" apologist, as most Lewis fans think. Rather, Lewis anticipated a recent approach in Christian apologetics that focuses on appealing to desires, rather than "proofs" or intellectual reasons, and seeks to show how the "big picture" of Christianity makes sense because it explains better than any alternative worldview the totality of human experience. Again, readers will have to make up their own minds whether McGrath is correct in this reading of Lewis. Some may think it reflects McGrath's preferred apologetical approach more than it does Lewis's.
As McGrath's endnotes make clear, this book is the fruit of wide reading and deep scholarship. He does nod in places. For instance, there is no discussion of the "Great War" between Lewis and Barfield in his chapter on Lewis's philosophical views in the 1920s. He gives no evidence of having read Lewis's lengthy unpublished "Summa," where Lewis defends a kind of Bradleyan objective idealism and an idealist/Kantian ethic at great length against Barfield's anthroposophy. (McGrath mistakenly describes Barfield as a "theosophist" (152)). For a fuller and more accurate account of Lewis's philosophical development in the 1920s, readers should consult Adam Barkman's fine book, C. S. Lewis and Philosophy As a Way of Life.