I was going to save this for an Aperiodical Round Up but it’s such a good thing I thought I’d post it straight away. Project Gutenberg has moved on from offering just plain-text transcriptions of books: volunteers have been outstandingly generous with their time and produced LaTeX versions of many maths books, producing versions that are considerably more readable and resemble the original editions much more closely.
Not all the books in that list have been converted to LaTeX yet. Of those that have, GH Hardy’s A Course of Pure Mathematics leaps out as a good place to start. Compare it with this book still in HTML format to see the difference.