Why are Christian books not Public Domain?

Because the Bible specifically commands it! :-)

1 Corinthians 9:13-14, "Do you not know that those who minister the holy things eat of the things of the temple, and those who serve at the altar partake of the offerings of the altar? Even so the Lord has commanded that those who preach the gospel should live from the gospel."

On a practical level -- and I don't want to get into politics here, but I think this is pretty elementary economics -- you often hear people say how much better it would be if such-and-such was free, how then everyone could get as much of it as they wanted or needed, how the poor would have as much access to it as the rich, etc. This is not just incorrect, but the exact opposite of the truth. If something is required by law or custom to be given away for free, then no one gets very much of it at all, because the supply dries up.

Suppose that tomorrow Christians all agreed that all Christian books must be given away for free. The immediate result would be that the number of Christian books published would plummet. I suppose there are some writers who hope to get rich by writing Christian books, and some who see writing as a more pleasant job than digging ditches or whatever. You might say that these people cannot be very sincere in their faith and we're just as well to not have them writing anyway. (Though someone could write a good book from bad motives.) But consider event the truly dedicated writer, whose over-riding goal is to bring people to Christ or to help Christians better live their faith. Still, if he is not paid for his books, then he must get some other job to support himself and do his writing in his spare time. This is not easy. Many would never manage to finish their books. Some would do less study or research or editing to get it done and out. At best a writer could not complete as many books.

I suppose some rich person might support writers. But how many generous rich people are there out there who would take on such a project? And if we ended up with a small number of rich people sponsoring all the Christian writers, they would presumably only support writers whose ideas they agreed with. Maybe they'd even pressure writers to change the content of their books to get support. Even if this rich patron didn't actually say to writers, "Change your book to support this doctrine that I believe in or I won't support you", still, if writers figure out that only authors who espouse certain doctrines get support, they might conclude they have no choice. I think that would be a very dangerous situation.

Disclaimer: I have published a Christian book and I do not give it away for free. I've also published a computer book and I don't give that away for free either. I haven't sold anywhere near enough copies of either to enable me to quit my day job. So maybe I'm the worst of both worlds: I don't give my book away for free to help Christians regardless of their financial situation, but I also don't make enough money from it to be able to devote myself full-time to Christian ministry. On the other hand, the reason why I haven't written more books is because I don't have time while trying to work a full-time job and raise three kids as a single father.

Both of the currently existing responses answer the question well. This response looks specifically at some of the costs associated with modern book publishing.

Book publishing is a massive industry, and it takes a significant number of people to bring a book to market. There are authors, editors, marketers, executives, administrators, accountants, lawyers (copyright & otherwise), HR departments, clerical staff, printers, supply chain logistics companies (distribution centers, warehouses, trucks, etc.), individual booksellers, and more. Each of them is integral to the process of getting content from the author's thoughts to the customers' shopping carts in a modern economy, and each of them needs to cover their own costs and make a profit, so that the people involved can pay their mortgages and put food on the table. Think about the number of people necessarily involved in producing a title that sells several million copies and is eventually translated into multiple languages for distribution around the world.*

When we do find cases of contemporary resources being provided gratis, it usually has been made possible by a sizable donation contributed by a wealthy individual or a financially successful ministry, sometimes in the form of the author's own ministry selling the item at or below cost. As economist Milton Freidman was fond of saying, "There's no such thing as a free lunch" -- that is, even when something is nominally "free," it took work to produce it, and somewhere, somebody either donated their own work, or payment was made by a benefactor on behalf of the end recipient.

It's also worth taking note of the extent to which the "e-reader revolution" -- the Kindle, Nook, etc., as well as fully functional tablet computers -- is causing massive disruption in the publishing industry. Profit margins are being decimated, there's an ongoing antitrust investigation into Apple and Amazon re price setting on e-books, and on and on. Several magazines have recently moved to online-only format because it's the only way they can stay viable in the face of competition from blogs that have next to no overhead and can fund their efforts entirely though advertising; SmartMoney Magazine is one recent example. Online publishing has effected Christian teaching ministries in that it has made it possible to provide resources to the public "for free," while incurring significantly fewer costs than would be involved in publishing the same material in print form.

All of that being said, I agree with Mike's statement: Many of the best resources are long out of copyright, and are available "free." See, for example, the great resources available at the Christian Classics Ethereal Library, paid for by the generous folks at Calvin College. The majority of the resources there are centuries old. It says something about the quality of a book when it is still being read and studied hundreds, even thousands of years after it was written, and in fact, we still know many of the authors by name -- Luther, Calvin, Spurgeon, Augustine, Aquinas, Josephus, Tertullian, and so on.

* If it's not too far off topic, here's a fascinating video where economist Milton Freidman explains how free markets encourage peaceful cooperation between many different people to create products that none could produce on their own: The Lesson of the Pencil. If you watch the video, just think about a book instead of a pencil; the principle still applies.

Argument in Favor -

It produces better quality work. If Christian authors have to support their families by themselves, writing becomes of secondary importance to them.

Paul was supported by the donations of various Churches on his trips.

2 Corinthians 11:9 ... the brothers who came from Macedonia supplied what I needed.

2 Corinthians 8:3 ... they gave as much as they were able, ...

Without their help he might not have been able to focus on his preaching work as well as he did.

Argument Against -

Publishing books and magazines has become a lucrative business.

John 2:16 Then he told those who were selling the doves, "Take these things out of here! Stop making my Father's house a marketplace!"

It's not uncommon to see Christian authors and ministers go around in private jets asking for charities for the Lord's work. Maybe hidden somewhere in there is a clue to your question.