they've damaged or gotten rid of them or are just so stupid they don't know what they did with them. send him a polite but firm letter by return receipt, restricted delivery, so you have a copy of their signature that they got the letter demanding you books to be returned within 3 days of the receipt of your letter as they are yours and you require the return or you will have to take legal measures to get them back. you will get some kind of answer. probably "i don''t know" at which point you will have to demand they buy you new ones.be prepared to go to small claims court if they refuse. sorry, but some people just aren't good friends and we find out the hard way bu losing our possessions.