What is the perferred material for a TT shelf?

Depends on the type of suspension you have.  If it is the now typical solid (or mildly isolated) suspension then hard maple butcher block makes an excellent base.  Other woods may work too, but hard maple is the most recommended.  Using harder materials like stone, glass, or maybe acrylic provides less damping and can negatively impact the sound.  There are of course folks who will insist the harder materials work fine, but you takes your chances....  Now if your turntable has a spring suspension you may have more choices, the springs help isolate the table from the shelf material, though I would still suggest maple. 

sorry for the misspelling. If I go solid wood, does it really matter what wood: maple, cherry or walnut? 

My current solid and my previous suspended table both benefited from being spiked to a 2" thick maple slab isolated (with those ubiquitous) cork/felt constructs from a shelf of baltic birch plywood  screwed down tight to a pair of wall hung cantilevers. 

Improved texture and tuneful bass. The maple acts as a sink, an engineer friend tells me and the lightweight shelf dissipates the energy to the wall.

It might be true. At any rate, there is an audible improvement.

That would depend upon the TT you are using.

Sans that info any replies are clueless.

You guys have me thinking that I may have chosen the wrong material, or perhaps the wrong decoupling method.  Am I better off with a large set of spikes under my acrylic than I am with the Herbies Audio Lab Grungebuster Dots?