I had a quick look at this recently when I noticed some ITunes gift cards on sale at a 20% discount. I have never bought an IBook but thought it may be a useful way to get a 20% discount on some titles. However, I thought I would first check to make sure I would be able to use it on my reader. I discovered that, as pdurrant pointed out above:
"There was a tool called Requiem, but the latest version for iBooks only worked with iTunes version 10.5.x."
About 20 minutes of searching failed to find even a single copy of this old tool available for download. I simply did not care enough to waste any more time on it. Even if it was available, and I installed an old version of ITunes, I don't like the chances of it removing DRM from current IBooks. I did not therefore buy an ITunes card and have no intention of buying any IBooks.