Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
Metadata is needed to make this work. First of all, the books or other resources need to be marked up consistently; that's where sound XML practices come in. But having good markup at a granular level isn't all you need; you also need metadata. Much metadata, of course, is relevant at the title level: that is, it applies to everything in the book. But to use metadata effectively for subdividing content, it's important to have IDs in the XML at the granular level. These granular IDs (at the chapter, section, and sometimes even the paragraph level) let you locate the chunks and point to the chunks. You can then associate metadata with those IDs. That lets you maintain the metadata separately from the documents themselves, which is essential given metadata will evolve over time.