I was recently contacted by Achilles Kostakis; a young 3D modeller who is looking to beef up his portfolio. I agreed to help him promote his services if he would offer his services for free to the first two of you to contact him, mentioning my blog, and he has kindly agreed. You can contact him by email on kwstakis1[at]outlook.com.
You can see below a few samples of his work. As you can see, he can make great 3D mockups of your books, or design something from scratch for your book cover.