Carl Friedrich Gauss’s textbook, Disquisitiones arithmeticae , published in 1801 (Latin), remains to this day a true masterpiece of mathematical examination. .
Gauss totally revolutionized mathin in general and the branch of number theory in particular with this book at the tender age of 21. I give it a 5 star rating for it's historical significance. However, it is very obtuse for the modern reader, and by no means suffices as a textbook for mere mortals. The most exciting result in the book is probably the law of quadratic reciprocity. Gauss gets the reader there, but langorously, first developing individual proofs for each of the low-primes, before establishing the general case. This is either exciting mathematics or excruciating depending on how much you enjoy following Gauss's thought processes. Most of the books is devoted to quadratic forms which are beyond my pay-grade and ability to comprehend.
This is the "Elements" of number theory. Gauss collected together many known results and techniques, and contributed a bunch of his own. Very elegant and imaginative. No exercises, unfortunately. :o)
I could not read even the 20% of the book, I could not follow almost nothing of the results of Gauss and the writing style is just too dense and trust me I have read Principia from Newton from the first line to the last and the Pentateuch, is not a problem of constancia. I think maybe Gauss was a brilliant mathematician but he was an awful writter. (2 stars at this moment, if I success in the reading I will change it).