* High level architecture (what pieces talk to each other, how they communicate, what they do)

* A list of 'here be dragons' areas, places where the code is gnarly

* A bug database, which will give you history of decisions

Then I'd ask the manager/product owner for a requirements document (or documents), as that should be something a non technical person can/has put together.

Frankly, though outside the scope of your question, I would also look at hiring one of the departed senior devs on a contract basis, just to ask questions of. I have offered this for the past few jobs I have left and there are times when 15 minutes of my time could save a few hours of investigation.