Nowadays, book printers mix different fonts. Aa example of this is Springer, which writes in Times New Roman but takes letters g, v and greek psi (I don't know if more) from computer modern
(i think).
How can I do the same? Hoe can import a single letter from another family font?
The closest question I have found is Unicode selection from other font but I don't know understand anything, and import symbols from other packages seems me not be valid for this case.
SOME EXAMPLES:
from Hussemoller's Fibre bundles book.
from A. O. Barut, R. Raczka Theory of group representations and applications, Polish Scientific Publishers distribution by Ars Polona (1980).
EDIT.
Talking yesterday with @cfr I notice I must make two changes:
1.- Certainly, I was thinking about use these import glyphs only in math mode. However, thinking better later, I realize that Springer also uses the ''imported'' glyphs in theorem-like environments (see the 'v' oth Theorem 3.2).
2.- When I post this question I was sure they import glyphs from another family fonts (some people in the typography.guru forum confirmed me that). However, some days later I discovered this PDF, shows that, at least in the case of Times New Roman, there was extra glyphs similar to other fonts. I haven't found the same for Barut's book yet, but it seesm me reasonable to think something similar happens and they don't import extra characters, but they font has extra glyphs.