I am writing a research paper which is concerned with Euler's book 'Introductio in analysin infinitorum'. May I refer to it as "the 'Introductio'"? And would it be wrong to simply refer to it as "Introductio"? Somehow the latter sounds wrong to me. Maybe it is because there are lots of "introductios" and therefore I feel the need to disambiguate using a definite article.
Also, to further generalise the question, is it the same if the title is given in English e.g. 'Introduction to the analysis of the infinite'? And can the same rules be generalised to similar book titles e.g. 'Foundations of Differential Calculus', 'Principia Mathematica'?