TITLE: Textbooks, lecture notes, and articles from arXiv for undergraduate students
QUESTION [13 upvotes]: I have found some interesting textbooks and articles on arXiv, such as the following one, that are accessible to an undergraduate student:

Course of linear algebra and multidimensional geometry, by
  Ruslan Sharipov.

My experience makes me believe that in arXiv there may be not only research papers, but also plenty of freely available interesting textbooks, lecture notes, and articles that can

give insightful explanations of topics that are commonly found in an undergraduate degree program;
but also enrich and broaden the mathematical culture of an undergraduate student.

Could you point out some good examples of such material?

REPLY [2 votes]: For people interested in Lie Algebras and Representation theory, Brian Hall's notes on the topic are available on arxiv.