TITLE: Good introductory book on geometric algebra
QUESTION [53 upvotes]: The title of the question already says it all but I would like to add that I would really like the book to be more about geometric algebra than its applications : it should contain theorems' proofs. Just adding that I have never taken a course on geometric algebra. I'm a 2nd year engineering student, so a "beginner" book style will be very good!!! Also mentioning what would be the prerequisites for mastering the branch is appreciated. Thanks.

REPLY [3 votes]: This is a late answer, but the book in question wasn't published until 2019.
In my opinion, by far the best book about Clifford algebras and related things is Geometric Multivector Analysis: From Grassmann to Dirac by Andreas Rosén (and I'm not saying that just because I happen to know the author). It is aimed at advanced undergraduates, so it's maybe not completely “beginner style”, but he puts a lot of effort into explaining the ideas and the philosophy of the subject, and there are complete and crystal clear proofs of (almost) all the theorems. The book builds the theory from the bottom, starting with exterior algebras and Clifford algebras, and finishing with the Atiyah–Singer index theorem.