This paper discusses the problem of horizontal (nonhierarchical) navigation in modern educational courseware. We will look at why horizontal links disappear, how to support horizontal navigation in modern hyper-courseware, and our earlier attempts to provide horizontal navigation in Webbased electronic textbooks. Here, we present map-based navigation -- a new approach to support horizontal navigation in open corpus educational courseware that we are currently investigating. We will describe the mechanism behind this approach, present a system KnowledgeSea that implements this approach, and provide some results of a classroom study of this system. Categories and Subject Descriptors H.5.1 [Information Interfaces and Presentation]: Multimedia Information Systems - Hypertext navigation and maps. General Terms Documentation, Design, Human Factors. Keywords Horizontal navigation, electronic textbook, similarity navigation, concept-based navigation, map-based navigation, Self Organizing Map,...