Most of the textbooks I use are quite new. The old books are the exception.
The oldest book about mathematics I use is Hajós György: Bevezetés a geometriába, a textbook on elementary geometry (in the sense of Euclid). The first edition is from 1950, I have a copy published in 1960. (Edit: it seems there's a German translation.)
I'm also using Knuth's The Art of Computer Programming, does that count as old now? The translation of the first volume is based on the second edition, of which the original was published in 1973.
