I have great collection of mathematics and physics and some geochemistry books in my shelf. Physical books, as I much more rather read those. I order them overseas so they're all in English, not in my mother tongue, which means there's more accurate information on everything. They're mostly used, but scientific litters won't go old like that as you probably know :>
Then of course I've all my lecture notes and some additional texts, that my professors have provided, on my computer too, but my physical collection is what I'm proud of.