The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Ki-Tabo (listen to audio recording for precise citation), writes that if one finds a book that belongs to another person, and the book bears the persons name, stamp, or other identifying feature, the finder may not use the book. He is obligated to return the book to its owner, and until it can be returned, he must care for the book properly, and may not make personal use of it.
If a person finds a lost item and cannot return it to its owner immediately, he may give it to somebody he trusts to watch it in the interim. For example, if the finder is leaving on a trip and does not want to take the article with him, he may entrust it to somebody he deems reliable. Lost items differ in this regard from a "Pikadon" an object that was specifically entrusted to somebody to watch. If a person gives an article to his fellow to watch it for him, and the friend accepts responsibility, he is not allowed to hand the object over to somebody else to watch it in his stead, since the owner specifically entrusted him with the object. If the person is leaving on a trip and does not want to take the article with him, he may bring it to the Bet Din, and the Bet Din will appoint a reliable person to watch the item.
Summary: If a person finds a lost item that could be identified by the owner, he must watch the item in the interim until it is returned, and may not use it. He may, however, entrust it to somebody whom he deems trustworthy and reliable. This is in contrast to the case of an article whose owner assigned a particular person to watch it, in which case it may not be entrusted to somebody else.
