Merrill Markoe was the original Head Writer — and some would say "person who had most of the ideas which people credit to David Letterman" — for David Letterman's talk shows. She has certainly been woefully undercredited for his success, which may not be surprising. A lot of people just plain want to believe that all the funny comes from the person who speaks it aloud. (I once dated a lady who, in much the same manner, knew but did not want to believe that her favorite actors in action movies had stuntmen and that the stars did not actually do those spectacular feats themselves.)
Also, I suspect that Ms. Markoe has been shorted on recognition by some folks' unwillingness to accept that all those funny ideas came from a woman. But everyone who was around Letterman's shows in the early days seems to recognize how essential she was to all that success. Here's a recent interview with her.
And here she is picking some of her favorite moments from those shows.