"The Spirit of Catholicism" by Karl Adam

One of the best apologies for Christianity I've read yet. Goes much, much deeper than C.S. Lewis, in my humble opinion. And it's available for free from EWTN. Here's one of my favorite passages in a chapter on the Pentecost.

"Therefore it was not literary records, incontestable documents, which were the primary means of bringing the message of Jesus to men, but the broad stream of the uniform life of faith of the primitive Church, a life based on the preaching of the apostles and animated by the Holy Spirit. How could it have been otherwise? A living thing, in all its depth and in all its extent, cannot be comprised within a few written sentences. Only that which is dead can be adequately delineated in writing. The living thing is continually bursting the temporary form in which literature must perforce embody it. At the very moment that literature is endeavoring to arrest and fix it, the stream of life is escaping and moving swiftly on. Therefore all literature, and even the Bible itself, is stamped with the character of its time, and bears a form which, however vital its content remains, yet all too easily seems stiff and strange to later generations."

But by all means, read the whole thing.