What happened to the books?

It's a question I would like answered.
In  our household the Senior Cat would answer a question like that with, 
     "They've gone to look for the sheets."
That refers to the mystery of what happened to the two sets of sheets my mother bought many years ago. She had them delivered along with some other household items she bought at the same time. I actually took the delivery. I saw the sheets. (They remain firmly fixed in my memory because they were patterned - brown peacocks on one set and blue swans on the other. I would not have bought either.)
We still have the other household items. We do not have the sheets. We never used the sheets. I never saw them again. What happened to the sheets is a mystery.
I am also missing almost 300 books but I suspect that my mother gave those to the charity shop when they moved here and simply refused to admit.
But, much more seriously, I am wondering what has happened to some other books.
I was the librarian for a small but specialised library belonging to a group of which I am still a member. As I was also reviewing books in the same subject area I was putting those review copies (which was of course how I was paid to review) in the library. When I ceased being the librarian but continued to review the books I went on passing the books on. I also passed on other books given to me, some of which were new. They were always books I knew the library did not have and which I felt others could benefit from their being made available.
For several reasons I have not used that library myself since I ceased being librarian. I am no longer as familiar as I perhaps should be with what is in there. At one time I knew the location and contents of each book - easy enough when there are only hundreds rather than thousands. 
It was therefore of concern to me when a friend came to lunch on Wednesday and mentioned that she was asking the group to buy a certain book for the library. She said it wasn't there and had been told it was not part of the library. I know it should be there because, according to my records, it is a book I gave to the library. It isn't one I would have used myself but it was of sufficient quality that I felt it should be on the library shelves. I think the current librarian would have felt the same way if she had seen the book.
I am wondering what has happened to it. Inevitably I am also wondering what has happened to the other books I passed on to the library. If they have not, as intended, been placed in the library then I rather wish I had kept them for myself. I would have been quite entitled to do so and I could have ensured that others had access to them if they asked - after all my own library contains many books on the same topic and they are frequently used by others.
And, if the book in question is not there where it should be, then I will always wonder...has it gone to look for the sheets?