We study a collection of heterogeneous XML databases maintaining similar and related information, exchanging data via a peer to peer overlay network. In this setting, a mediated global schema is unrealistic. Yet, users/applications wish to query the databases via one peer using its schema. We have recently developed HepToX, a P2P Heterogeneous XML database system. A key idea is that whenever a peer enters the system, it establishes an acquaintance with a small number of peer databases, possibly with different schema. The peer administrator provides correspondences between the local schema and the acquaintance schema using an informal and intuitive notation of arrows and boxes. We develop a novel algorithm that infers a set of precise mapping rules between the schemas from these visual annotations. We pin down a semantics of query translation given such mapping rules, and present a novel query translation algorithm for a simple but expressive fragment of XQuery, that employs the mapping rules in either direction. We show the translation algorithm is correct. Finally, we demonstrate the utility and scalability of our ideas and algorithms with a detailed set of experiments on top of the Emulab, a large scale P2P network emulation testbed.