When it comes to Civil War book collecting, Paul Taylor always has something interesting to say. Check out his latest commentary on the current state of the lifestyle. [While you're at it, take a look at his new book about Civil War Detroit titled "Old Slow Town". I have a copy and it looks good.]
My own collecting bug is geared toward modern books -- from the Centennial period to today -- and I've always considered it an intellectual and aesthetic investment rather than any kind of financial one. Paul's comment that the peak period of collecting was the mid-1990s, before internet tools broke down the traditional forces propping up prices, has the ring of truth to it. I recall a dealer remarking in a 2005 article somewhere that a good Civil War book collection valued at $200,000 in the 90s was only worth $40,000 a decade later. Going on ten years more, the value gap has undoubtedly only widened further.