Well, fortunately for me about half of my books are in the home office and not every visitor goes in that room, there are 5 book shelves along the main downstairs hall though that is readily seen by anyone, the main bookshelf I have trouble like that with is the one that is loaded with health books, so I have a ready made excuse with that, saying oh, sorry, I refer to that all the time, can't lend it! And smile sweetly.
The rest of the shelves are mainly history, which practically no one else is interested in.
And frankly I'm to the age where if they don't like it, they can lump it. Anyone who knows me well knows I hate to lend books and wouldn't ask.