I speak from experience: You need to get a handle on this now, otherwise you stand the risk of the theft escalating.
My brother and I, who live together and pool our books, had multiple books dissappear. We bought FIVE, yes FIVE copies of Magic of Faerun b/c our Magic of Faerun kept walking away. Since we played at our place, the books never left the house. We also lost the FRCS, 3 Mongoose Quintessential books, and probably 10-15 others over the course of 2 years. Since my brother and I both worked at a game store, I'm betting the thief figured that we could afford to replace our "losses" with our employee discount.
We told the group, pleaded with them to bring back books, and then grew somewhat cynical. We even set up a box that someone could sneak up and deposit a "borrowed" book without being seen. None of these measures worked.
Nowadays, beyond getting rid of the thief, we write our names all over our books to the point of being obnoxious. All three page edges, initials/name on the front cover, back cover and spine, inside cover, outside cover.
This may sound silly, but you need to fix a price on your friendship. How many books are you willing to lose? Take it from someone who suspected but could never prove that someone was a thief.
This much is true: Calling out someone you suspect is pointless without proof and will lead to much yelling and hurt feelings. Setting up surveilance is not a bad idea as it might seem. If you show a video of someone stealing a book to the rest of the people in the group, there is no way the others will sympathize with the thief if your books are marked in an obnoxious fashion. Yeah, it might feel bad to put stickers or other very visible markings on your books, but I would encourage you to do so. Basically, you make it so there is NO FREAKING WAY someone can take a book by "mistake".
Asking the group for ideas is fine in principle, but really, what do you expect? You will confuse and stump your players. Take matters into your own hands, don't call the police, and don't beat up anyone!
After other measures are taken, you can escalate matters. Make sure you clean your place up completely to ensure the book did not get stashed somewhere as a practical joke gone too far (I've had that happen too). Thanks for listening.