Catholic Church Architecture and the Spirit of the Liturgy. Though it looks like a coffee-table book, it is a feast for the mind and heart as well as the eyes, and completely complementary not only to David Clayton's work, mentioned elsewhere on this site, but to the many brilliant books of Scott Hahn (who wrote the Foreword) and the "new wave" of liturgical writers inspired by Ratzinger's The Spirit of the Liturgy to recover symbolism, cosmology, and the principles of sacred tradition.
The key principle is this: “Architecture is the built form of ideas, and church architecture is the built form of theology.” “As go the ideas, so goes the architecture.” No wonder things went off the rails in the 1960s. “We call things beautiful when they reveal their ontological ‘secret’, the invisible spiritual reality of their being as objects of understanding.” What makes this book much more than an exhortation, or a manifesto, or a philosophical treatise, is the precise and careful thinking that has gone into the rules of beauty, which follow in large part from this definition. We may “like” a church that reminds us of a comfy living room or Swiss chalet or an aircraft hanger, but that doesn’t make it “beautiful”; it doesn’t make it look like a “real church”. Thanks to this book, future generations are more likely to have real churches in which to worship.
If this subject interests you, you'll probably also enjoy Jean Hani's The Symbolism of the Christian Temple and Stephen J. Schloeder's Architecture in Communion.