There is an impressive demand for free Christian books. Luckily, there is also an impressive amount of free Christian books available on the web.
Bibles

The first anyone interested in free Christian books might look for are bibles. Bibles are often for free. Many of the most common versions of the bible can be found on the web.
Bibles are often for free. This counts for instance for the text of the King James Version (KJV) of the Holy Bible (also called the Authorized Version (AV) by some). This version is in the public domain. You may copy and publish it freely. It is available at Holybooks.com. The new international version (NIV) bible is also widely available. For instance, Turnback to God is a fine site for anyone interested in a free niv bible download. The catholic bible is also in high demand. Good copies of the Catholic Public Domain Version (CPDV) can be obtained from Sacredbible.org.
Websites with free Christian books
There are zillions of websites with free Christian books. Not all of these are of a high quality and the seeker must beware of crappy websites with questionable content. It would be too demanding to list all the good sites here. Therefore, the following description is limited to some websites that are interesting for the Christian seeker of free ebooks.

One easy way to find Christian books is to visit this website’s theme page about websites with free spiritual books. Click some links that lead you to general free book websites with many bookshelves well-filled with high-quality spiritual and religious books such as Project Gutenberg, Open Library or The Online Books Page. Then browse for Christianity of Christian books in particular.
Another way is to go to some Christian wwebsites with many free books. For instance, like the name says, Turnback to God is a website for people who want to turn back to god. It is maintained by Georgy N. Joseph, who, as a former sinner, identifies himself with the The Prodigal Son in the Bible. After attending a few programs of Jesus Youth, he now finds that he is leading a much better and sin-free life. On his website he would like to share all the thoughts that attracted his attention during his life change. The site has a lot of famous free Christian books to download. Some of these contain debatable religious thoughts. I think it is interesting for people who, just like Georgy, just changed or are changing their lives towards adopting a Christian lifestyle.
Another interesting site is the NTS Library.com, an Online Christian Library of Virtual Theological Resources. This website grew out of a Christian online referral services library project offered by Northwestern Theological Seminary, for which the NTS stands for, and the Northwestern Christian University. They refer to themselves as a leading worldwide Christian Internet community learning resource. Whatever they are, they have many interesting free Christian ebooks to download.
Another site to browse for free Christian books is Plough. An independent religious publisher with a wide array of offerings, The Plough Publishing House has been the literary mouthpiece of the Bruderhof, an international movement of Christian communities, since the 1920s. Plough is well known as the supplier of choice for churches, religious non-profits, and other organizations looking to purchase large quantities of low-priced books for distribution at retreats, workshops, conferences, prayer breakfasts, and the like.
Of interest for the protestant seeker of free Christian ebooks is the website Monergism.com. This is an Internet based non-profit resource ministry whose mission draws on the words of Martin Luther on Monergism in his Small Catechism: “I believe that by my own reason or strength I cannot believe in Jesus Christ, my Lord, or come to him. But the Holy Spirit has called me through the Gospel, enlightened me with his gifts, and sanctified and preserved me in true faith.” They have many interesting free Christian ebooks for download.
Finally, for the seeker of free Christian books, Holybooks.com is absolutely a great site. Like the name says, they have all kinds of holy books for free. this includes a substantial amount of classic Christian books.
Free Christian fiction books
Another category of possible interest are free Christian fiction books. These include free Christian novels, which are usually in high demand. Such books can be very inspiring for the sincere Christian seeker. Some of the aforementioned websites also offer such free Christian fiction books such as Plough and Turnback to God. Nevertheless, it is not easy to find such books since they are usually well hidden between the other categories of books. The best way to find these books is to browse high-quality websites with free ebooks such as Project Gutenberg, Open Library or The Online Books Page. And next to search for your author of choice. If you need names of famous authors of free christian ebooks, you might want to consult a wiki page or the Christy Awards website.