Sorry, but the book you wanted to get digitally through iBooks has been banned from the store by Apple due to explicit content.
You can read about the situation on the Comic Book Legal Defense Fund website. (Donate to the CBLDF, by the way. They are awesome!)
If you think Apple has made a mistake in rejecting adult books intended for—and clearly marked for—adults, then you can provide feedback—of type, “Other”—here.