Just putting aside any price considerations...
I always keep 3 copies of the books I like, say
1 hard copy of Kolmogorov and Fomin
1 kindle version
1 pirated version downloaded on the net
Do you preferer books or e-books?!
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I always keep 3 copies of the books I like, say
In all seriousness, I do often have 2 copies of technical books: both a real physical hardcover (which I use when I want to do serious distraction-free reading, and write in the margins) and a pirated pdf (which I use when I don't have access to the physical book).