H.M. Edwards in the preface to his book on the Riemann Zeta Function, summarises his philosophy on learning Mathematics:
...I have tried to say to students of mathematics that they should read the classics and beware of secondary sources
In trying to learn more, I feel that I have accumulated a bookshelf full of secondary sources which I have left largely unread. So I would like to take heed of Edwards's advice and read some classics for the wonderment of it and for the respect I would gain for mathematicians of the past. I would like to exercise creativity over rigour, at least for a little while.
But this begs the question: what are the classics? And this is what I hoped to ask about.
Which primary sources do you feel are suitable for self-study? Naturally this is a broad question, but people must have their favourites, and I am hoping for some recommendations on the basis of these personal preferences.
For introductory Number Theory, you could go with Gauss, Disquisitiones Arithmeticae. Don't worry, you don't have to read Latin, it is available in English and other living languages.