As one of the most complex and advanced forms of Internet information systems, digital libraries serve as an increasingly important channel to a vast array of information sources and services. However, from the standpoint of satisfying human's information needs, the current digital library systems suffer from the following two shortcomings: (i) inadequate strategic level cognition support; (ii) inadequate knowledge sharing facilities. In this paper, we introduce a two-layered digital library architecture to support different levels of human cognitive acts. The model moves beyond simple information searching and browsing across multiple repositories, to inquiry of knowledge. To address users' high-order cognitive requests, we propose an information space, consisting of a knowledge subspace and a document subspace. A formal description of the knowledge subspace for knowledge sharing and dissemination, as well as mechanisms for constructing the two subspaces, are particularly discussed. Such an enhanced information space extends the traditional role of digital libraries as information provider to information & knowledge provider. Some distinguished features in comparison with the traditional knowledge-based systems are also discussed in the paper. are there any important features missing from the current DL systems? To answer these questions, let's first look at a scenario on the use of a DL system.Kooper works in a city plan & management office. He is investigating the precaution policy against flood this year. If there will be a flood in the coming summer, necessary actions (e.g., strengthing the embankment, resource allocation, etc.) must be taken now to prevent the city from sustaining losses. According to Kooper's previous experience, it seems that "A wet winter might cause flood in summer." To confirm this pre-conceived hypothesis, he logins on a DL system to request information talking about the reasons of flood. The DL system returns a number of articles. He browses through the returned article list and selects three articles that look most relevant. All of the three articles mention that "A precursor to the flooding in summer is a wet winter." To assure this information is also valid for the area where Kooper lives, he accesses the DL again to ask for documents reporting river flood in the city before. From the articles returned, he gets to know the latest 3 severe flood that happened in 1986, 1995, 1997, respectively, in the city. He then navigates to the meteorological repository of the DL, and accesses the weather information of the city during the winter time of these three years. He notes a tight correlation between wet winter and flood summer. Based on the information obtained from the DL, Kooper is pretty sure now about his prior flood prediction assumption for the city. He logs off the system. Having experienced a very wet winter this year, Kooper decides to draw up a city flood precaution plan immediately.From this scenario, we observe that a number of deficiencies exist in current DL systems. Bel...