Font in "Geometric Invariant Theory" by Mumford

The font is Monotype’s “Old Style Series 2” which was one of the main fonts used for mathematical publishing along with Monotype Modern 1 & 7, Imprint, Times and Baskerville (in the US, one could also add Monotype Modern 8 and Century Schoolbook). Here’s a showing of the alphabet in the Monotype Mathematical Sorts List booklet by Arthur Phillips (1956):

You’ll notice that the italic f  is different, but it was quite common at the time to substitute an f  without kerns to ease the setting process. As often, they also used a generic set of numbers (from Monotype Modern, probably) which does not match the font.

Here’s a close-up of some characters from Mumford’s book (page 34) which are typical of Old Style Series 2 and allows to identify it:

Monotype did a digitalisation of it as Monotype Old Style, but there’s no LaTeX package for it and no companion math font.

Have you tried the WhatTheFont website?

I suggest to use identifont.