Yes, it does mean that, one book a month. With no due dates, you are basically getting a free book, although I would be surprised if they would let you keep the book forever - there must be a cut off date somewhere. On the surface at least, it looks great for readers, but not so good of course for the author. Amazon do have a royalty programme for authors whose books are enrolled in this programme, but you have to agree to sell your books exclusively through them, which most authors are understandably loathe (I personally wouldn't do this), hence the fact that not many authors have bothered to enrol. For £49 a year, as others have said, you might as well just buy the books anyway.