Peer-to-peer networks have been identified as promising architectural concepts for developing attractive search scenarios across digital library collections. Digital libraries typically offer sophisticated search over their local content, however, search methods involving a network of such stand-alone components are currently quite limited. We present an architecture for highly-efficient search over digital library collections based on structured P2P networks. As the standard single-term indexing strategy faces significant scalability limitations in distributed environments, we propose a novel indexing strategy-- key-based indexing. The keys are term sets that appear in a restricted number of collection documents. Thus, they are discriminative with respect to the global document collection, and ensure scalable search costs. Moreover, key-based indexing computes posting list joins during indexing time, which significantly improves query performance. As search efficient solutions usually imply costly indexing procedures, we present experimental results that show acceptable indexing costs while the retrieval effectiveness is comparable to the standard centralized solutions with TF-IDF ranking.