FIG. 1 is a front perspective view of an embodiment of a modular bookshelf system showing our design; FIG. 2 is a front orthographic view thereof; FIG. 3 is a rear orthographic view thereof; FIG. 4 is a top orthographic view thereof; FIG. 5 is a bottom orthographic view thereof; FIG. 6 is a left side orthographic view thereof; and, FIG. 7 is a right side orthographic view thereof.