Thankfully Bearss is still alive and kicking. It is my understanding that anything written during employment with the National Parks Service is the intellectual property rights of the NPS. No royalties to Bearss or Maggie during their lives or residual income afterward.
Say Bearss wrote a more magnificent and encompassing study of Vicksburg with much better maps (I'm thinking Bradley Gottfried style maps showing which regiment fought where and when) in 2019, should Bearss then cross the Styx. His wife Maggie gets the rights. If Maggie predeceases Ed, then Bearss' childrens inherit them. In no kiddies, then Bearss' parents gets them (of course they're in the afterlife). If no parents, then his siblings and if no siblings, then offspring of the siblings (nephews and nieces) in equal shares. Of course, this is subject to his will/trust or Maggie's will/trust. Ask Eric Wittenberg, he's an atturnii.
I haven't checked the copyright date, but if Ed wrote them after leaving the National Parks, it's his. Then again, he could have sold it to Bob Younger (Morningside Press) and whomsoever bought Morningside Press from the Younger has them.