MY 19x18 bedroom has a tiled bathroom. The tile already has a marble transition piece installed. My flooring will run parallel with the bathroom doorway. Can I lay my flooring right up against this marble transition piece or do I need to leave some kind of expansion gap? Because the marble transition piece is 1/4 higher than both the tile and wood flooring, my options may be limited if I indeed need an expansion gap.

What type of flooring is it....species, width, thickness, solid, engineered? House location? Subfloor?

Sorry. Solid 3/4' pre-finished oak - 5' wide flooring (Lumber Liquidators Bellawood Select grade). Subfloor is 3/4' plywood. House is in a very humid summer climate in New England (in fact my previous question was regarding installing the flooring after almost two weeks of solid 80% relative humidity). Wood has been acclimating now for three weeks or so.

House air conditioned? Have you been checking the moisture content of the flooring? Where in New England?

Expect some nice shrinkage/gaps when the heating season arrives unless you can keep that Rh over 50% Probably still gap some sitting in 80% Rh.

You're basically acclimating to a summer condition, not an average. Average should be the goal.