This quote from the book describes Ivan's view of his drawing room in his minds eye
"Looking at the yet unfinished drawing room he could see the fireplace, the screen, the what-not, the little chairs dotted here and there, the dishes and plates on the walls, and the bronzes, as they would be when everything was in place."
Most of this is crystal clear except a what-not???? So I googled it!!
A what-not is a piece of furniture derived from the French étagère, which was exceedingly popular in England in the first three-quarters of the 19th century. It usually consists of slender uprights or pillars, supporting a series of shelves for holding china, ornaments, trifles, or what not, hence the allusive name.
They could be plain or fancy as these pictures show and normally made from mahogany.