This gem from the history of mathematics is more recent. It was published in 1940 by British mathematician G.H. Hardy. The book/essay was written when Hardy (then 62) felt that he no longer had the ability to contribute to the field of mathematics. A main theme in the book is concerning mathematical beauty, and he believed that the most beautiful mathematics was that, which had no application! Luckily, this book is also in the public domain, and you can read it in below (or download the pdf):

A Mathematician’s Apology