When learning mathematics, I think it is better to learn the motivation and the purpose of a particular maths idea. I believe such things are best found in the original paper which proposes the idea. How could I find such papers?
For example, I am currently learning group theory. How could I find an old paper which proposed the theory?
Harold Edwards agrees with you about reading original sources. His book on Galois Theory includes a translation of the seminal papers on Group Theory and an excellent account of the mathematics from both the original and the modern perspective.