They've swept away all the other marks of their feet from the carpet; but whoever did the sweeping was too slack to lift up that book and sweep under it.
This footprint, however, is not of great importance, though it is corroborative of all the other evidence we have that they came and went by the garden.

There's the ladder, and that table half out of the window.

Still, this footprint may turn out useful, after all.

You had better take the measurements of it, inspector.

Here's a foot-rule for you.