Oh my, I'm sorry to say that this happens to me, too. And it is SO FRUSTRATING!!... makes a Christian woman want to swear!! I do refrain myself, though. Do you keep your books in more than one place? Have you taken them all off the shelves and restocked them? I don't know what else to suggest. I hope you locate them because I'd hate to have to purchase duplicates.
I have been in an organization mode for about a year now. We winter in one house and summer in another 500 miles away so it is essential that I know where things are and.... things can be a burden at some point. I've been there. I now have a quilting organization book that tells me where things are. I know.... I'm a little OCD (my kids say a lot OCD) but it works for me, finally. I don't spend near as much time 'looking for something' as I used to. And as I get older, I'm also giving some stuff away that I won't ever use.