At some point, Jake will start crossing paths with historical figures.
It’s a definite that he’ll collaborate with FDR and Gen. George S. Patton to take down Hitler.
But, and I’m not sure yet, but he might also have encounters with mobsters like Bugsy Siegel, Lucy Luciano, Meyer Lanky, etc.
He may even work with J. Edgar Hoover on a case.
Question about finding/using pictures of historical photos.
Do these pictures just belong to the ages? FDR was our president so are we entitled to use an FDR photo whenever we want?
What about generals? Mobsters?
What about Hitler? I don’t want to be sued by Hitler.
The question is, can historical photos of famous folk be grabbed and used? If not, is there some kind of repository or place that permission can be asked to use such photos?
Bookshelf Battle 
My wife is a photographer, and she will tell you that the photo always belongs to the person who took it or who now owns the rights to it (or the estate of said people).
I rip stuff off the web for my blog that is read by only ten people, but to put such pictures in a book may prove a little more of a minefield. Be wary.