I have experienced this for the second time now.  First time I thought it was just a mistake on my part.  Not so sure now.

I went through my public bookshelf  top to bottom to verify that I didn't miss removing anything from past deal offers.  And I came up with a few extra books that I thought where posted that weren't showing up.  I had done this once before and had the same thing occur.  So why are some of my books dropping of my bookshelf or just not posting when I think they have? 

If you are sorting your bookshelf by author, and the missing books don't have an author listed, that could be the problem.  Try sorting by title or by date posted.

I see 217 books on your shelf, but if I sort your shelf by author, I see only 215, so that's a good guess.

If the book you posted is a wish list book, it won't go onto your bookshelf (but you probably know that.)