[boomboomsubban]

It's rather famous. Written by Leo Szilard but signed by Einstein who later seemed to regret it. https://en.wikipedia.org/wiki/Einstein%E2%80%93Szilard_lette...

I don't understand why it would be in the Spy Museum.

[Uhhrrr]

Cold War relevance, maybe?

[Yoric]

As far as I remember, this letter started the race to nuclear weapons between the US and Nazi Germany. Lots of spycraft involved in that race.