I can't say I've ever read either of those books by Boyd, but he is a top notch scholar and author, so I think they would be worth your time if they sound appealing to you.
I never like recommending books to people unless I know their tastes, since it's a waste of time if you're not going to enjoy the book. So, I'm going to shoot buckshot and post a whole bunch and maybe one or two will be of interest to people. I'm only listing books I've personally read, but for most of these authors I'm sure their other works are good as well.
General/ComparativeThe World's Religions (
http://www.amazon.com/Worlds-Religions-Great-Wisdom-Traditions/dp/0062508113/ref=sr_1_2?ie=UTF8&s=books&qid=1282013898&sr=8-2) by Huston Smith: Published over 50 years ago as
The Religions of Man, the title and sections of the book have been revised to be a bit more PC (such as not calling tribal religions "primitive"). It's a great overview of the major religions of the world. It's objective, but still seeks the heart of each religion by focusing on their core beliefs rather than getting bogged down in details or debates.
What is Scripture? (
http://www.amazon.com/What-Scripture-Wilfred-Smith/dp/0800626087/ref=sr_1_1?s=books&ie=UTF8&qid=1282014644&sr=1-1) by Wilfred Cantwell Smith: An interesting look at the history of what the idea of "scripture" means today and how it has varied over history and cultures.
A New Religious America (
http://www.amazon.com/New-Religious-America-Christian-Religiously/dp/0060621591/ref=sr_1_1?ie=UTF8&s=books&qid=1282014885&sr=1-1) by Diana Eck: An outcome of the work of the Harvard Pluralism Project. Examines how America is effected by its relatively recent religious diversity, and how America in turn affects those religions that reside here.
Science and Religion (
http://www.amazon.com/Science-Religion-Introduction-Alister-McGrath/dp/1405187905/ref=ntt_at_ep_dpt_6) by Alister McGrath: Exactly what it sounds like. McGrath holds a PhD in molecular biophysics and a Doctor of Divinity from Oxford. I have the 1st edition of this book (which was much less expensive...good lord), but I'm guessing it's largely the same. He treats things pretty fairly, albeit only through a Christian perspective. Especially in his other works, he is a good counterweight to Dawkins, who was mentioned in a post above (it's worth it to read them both).
When Religion Becomes Evil (
http://www.amazon.com/When-Religion-Becomes-Evil-Warning/dp/0061552011/ref=sr_1_1?s=books&ie=UTF8&qid=1282016681&sr=1-1) by Charles Kimball: Not as hostile as it sounds. One thing that always bugged me was people trying to sweep the fact the religious violence occurs under the rug. Kimball points out that it's foolish to believe religion does only good or only evil, and attempts to examine the "warning signs" of religion leading to disaster.
Terror in the Mind of God (
http://www.amazon.com/Terror-Mind-God-Religious-Comparative/dp/0520240111/ref=sr_1_1?s=books&ie=UTF8&qid=1282017710&sr=1-1) by Mark Juergensmeyer: An inquiry into the relationship of religion and terrorism, from a sociological perspective. Very well done. More technical than Kimball, but more rewarding as well.
The End of Faith (
http://www.amazon.com/End-Faith-Religion-Terror-Future/dp/0393327655/ref=sr_1_1?s=books&ie=UTF8&qid=1282018039&sr=1-1)/Letter to a Christian Nation (
http://www.amazon.com/Letter-Christian-Nation-Vintage-Harris/dp/0307278778/ref=sr_1_2?s=books&ie=UTF8&qid=1282018039&sr=1-2) by Sam Harris: Okay, this one is hostile towards religion, but still well written. I say "hostile", but he doesn't rant, he just believes the stakes are so high we don't have time for political correctness. Further, with 62 pages of footnotes and a 28 page bibliography, his work has a sound academic base. I don't necessarily agree with everything he says, but if Christians want to see into the mind of an atheist, this is a good starting point. Just approach it as you would want an atheist to approach the Bible.
A Bit More PoliticalMoral Man and Immoral Society (
http://www.amazon.com/Moral-Man-Immoral-Society-Theological/dp/0664224741/ref=sr_1_1?s=books&ie=UTF8&qid=1282018795&sr=1-1) by Reinhold Niebuhr: Not too much theology here, but Niebuhr investigates the disparity between collective actions and individual morality. If it intrigues you, he builds a theology consistent with it in later works. A quick read by one of the greatest theologians of the past century.
The Clash of Civilizations and the Remaking of World Order (
http://www.amazon.com/Clash-Civilizations-Remaking-World-Order/dp/0684844419/ref=sr_1_1?s=books&ie=UTF8&qid=1282019058&sr=1-1) by Samuel P. Huntington: Not a book on religion per se, but most of the "civilizations" have some religious basis. The book's influence has waned since its publication, but it's a classic and worth being aware of.
What Went Wrong? (
http://www.amazon.com/What-Went-Wrong-Between-Modernity/dp/0060516054/ref=sr_1_1?s=books&ie=UTF8&qid=1282019359&sr=1-1) by Bernard Lewis: When the light of intellectualism in the west was little more than a flicker in the dark ages, the Middle East led advances in almost all areas of life. So why is it seen as backwards and oppressive today? A nice insight into modern Islam.
Jihad vs McWorld (
http://www.amazon.com/Jihad-vs-McWorld-Globalism-Tribalism/dp/0345383044/ref=sr_1_1?s=books&ie=UTF8&qid=1282019627&sr=1-1) by Benjamin Barber: Another classic along the lines of
The Clash of Civilizations. Highlights the paradox of the trend of globalization conflicting with increasing identification with small "tribal units".
A Little DifferentThe Varieties of Religious Experience (
http://www.amazon.com/Varieties-Religious-Experience-William-James/dp/0980060540/ref=sr_1_1?s=books&ie=UTF8&qid=1282020132&sr=1-1) by William James: A book on the psychology of religion by one of the men who shaped the field of modern psychology. Not as straightforward as some of the books listed here, it will take some effort to get through and you may need to do some research if you're unfamiliar with pragmatism. Don't let that deter you though, there's a lot of gems in this book.
Mystical Experience of God (
http://www.amazon.com/Mystical-Experience-God-Philosophical-Philosophy/dp/075461445X/ref=ntt_at_ep_dpt_4) by Jerome Gellman: Again, perhaps some background is required--I recall having some trouble with parts of it. A lot is packed into this relatively short book. I just threw it on here because of the discussion in other threads.
ExistentialismI love existentialist thought. It is intriguing and rewarding. It challenges and forces confrontation with probing questions about how you live your life. It encompasses some of the most brilliant minds of the past 150 years, both towering pillars of Christianity (Kierkegaard, Tillich), and some of the most violent attacks against them (Dostoevsky, Nietzsche, Sartre). Most of my absolute favorite works belong to this...and yet I'm extremely hesitant to put anything here. The problem is that there is a "price of admission". This is a body of thought that has been tremendously influential and it's hard to pin it down in just one or two works. Most of the authors wrote several works and assume you are familiar with all of them (Kierkegaard even assumes you've read what he wrote under pseudonyms, and Nietzsche was intentionally misleading to those that don't read deeply into his works). I think Tillich is one of the greatest theologians to ever live*, but he won't make sense unless you're acquainted with those before him.
If you want an introduction to existentialism as a whole, Walter Kaufmann is the best and Robert C. Solomon isn't bad. Avoid Sartre to start. Avoid Heidegger at all times (unless you're going to grad school in philosophy and like pain). If you make it past this point, we'll talk.
* Funny story: In Harris'
End of Faith, as he is pointing out how irrational he believes faith to be, when he comes to Tillich, he basically says "If all Christians were of Tillich's caliber, I would have no reason to write this book."