"Books that have become classics - books that have had their day and now get more praise than perusal - always remind me of retired colonels and majors and captains who, having reached the age limit, find themselves retired on half pay"

This quote by Thomas Bailey Aldrich is a comparison between books that have actually become classics and retired military officers. Aldrich is recommending that these books, like the retired officers, have had their day and are now praised more than they read. He is indicating that these books, like the retired officers, are no longer as appropriate or beneficial as they once were. This contrast is a tip of the short lived nature of popularity and success, and how rapidly things can end up being outdated and forgotten. Aldrich's quote is a reminder that nothing lasts permanently, which even the most successful and popular things can ultimately end up being obsolete.