Yea, I think there is room for growth. It seems to me that the packed PAX South show had a TON of new folks in the audience that haven't yet given D&D a try.
I've talked to retailers here and Volo's sold really well in retail.
Which is great news, as I would definitely be happy if it becomes the blueprint for future monster-themed sourcebooks. They really hit a nice sweet spot between crunch and lore in its design.
Somebody recently asked [MENTION=697]mearls[/MENTION] about whether we might see a book like 2E Faiths & Avatars in the style of Volos, and he said that was something he is considering long-term: I think Volos is a model they will follow in the future. I have my fingers crossed for Eliminster's Tour of the Planes, or something like that...
I think so. It is really strange. Just when I think the momentum is done or beginning to slow down I run into a whole new group of lapsed or brand new players chomping at the bit.
Slower release schedule means you can buy every book released.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.