Actually, it is a crown; if you look at the drawing on the table up close by adjusting the camera view, before you receive naminé's sketches, then you will see that there is one empty spot in a trinity set-up, and the other two are filled with a heart and some other symbol
(i can't remember what they are, but after I saw the cutscene where he completed the drawing I expected to see a nobody symbol, but on the picture of naminé's sketches, there was a crown.) I was expecting to see a nobody symbol, but I caught a glimpse of a crown instead, just before the floor disappeared. it'd make more sense if we could find a video, or a picture, but I couldn't find any scenes of the library, on the web.