Looking around the IAS website yesterday, I noticed that a selection of papers from Selberg’s archive is now available online, to be supplemented by a more comprehensive website. Mostly, these seem to be his transparencies and other notes for lectures he gave (eg., this one), rather than drafts of unpublished work. One interesting item is a transcript of an unpublished interview from 1989. Here is a quote I like (beginning after a discussion of the Chowla–Selberg formula and collaboration in general):
Yes, this was something, of course, quite different from what started it. It’s rather typical in many ways that in mathematics very often what you end up with has very little to do with what you start out with. You may start out trying to do something, and as you get into it and learn something either your attention may switch completely, — because you understand something more of the problem, perhaps what you had initially as a goal is quite impossible — or you may come across something as you are going along, quite by accident, that completely throws your attention in a completely different direction.
One can never, I think, predict where one is going when one starts out.
(p. 29 in the the first part of the interview)
Some things I also learnt or realized: Mordell was American (and not English); Selberg got the Fields medal well before he proved the trace formula (1950 against the first results in 1951–1953); Veblen used to organize wood-cutting expeditions in the IAS woods, and apparently Pauli was considered a dangerous participant with axe and saw (page 6 of the second part).