In case anyone is interested, here are the major releases for all three game systems sorted by type (new game [edition] / starter set, army book / codex / sourcebook, expansion or miniature wave release).

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The large increase of books around 2007 / 2008 was due to all the War of the Rings books that were put out in a relatively short period of time. That didn't work out so well... Otherwise things are remarkably stable, with the big difference being a shift from expansion-based releases to waves with no book to go with them.