A companion enhances the engagement of a particular book (or set of books). It does this by providing things relevant to, but missing from, the original works: corrections and updates, sources and commentary, prequels and sequels, photos and paintings.
Classic companions take the form of printed volumes. In the last decade, however, I assembled, on the Military Learning Gateway, on-line companions to some of the books I had written.
*The two books about the British Army in the First World War are linked to a single companion.
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