Be wide enough to connect ideas and deep enough to build with one of them.

Breadth and depth do different jobs, and neither works alone. Wide reading gives you the raw material for connections, since an idea from one field only becomes useful when it lands beside a problem in another. Depth gives you somewhere to put those connections, because you cannot build with an idea you only half hold.

Look at your own balance honestly this month. If you know a little about everything, pick one thing and go three levels further than is comfortable. If you only ever go deep, read something entirely outside your lane.