What do we have already?

Before creating a digital publication, one should carefully consider the information that already exists -- in some form or another -- and determine whether it is indeed sufficiently valuable for inclusion. Forming an understanding of the current content assets is called a 'content inventory'. Usually, it's a highly structured spreadsheet, where for each piece of content (one line) are listed:
Creating a content inventory is a meticulous job that requires one to systematically dig or hunt for content, including content that is not obvious to those that have been managing it for a long time. The process of creating a content inventory gives the information architect a deep understanding of the available data, and this is useful for almost any step in the design process.
The risk, particularly for very large bodies of content, is to be spending too much time listing every individual item. It may be necessary to deal with certain content in a more high-level way, only digging deeper for a subset of data.
The effort required for a content inventory is determined by the quantity and diversity of the content items and the detail required in the metadata.