This free e-book (at least when I discovered it) contains all one needs to get started and going. Plenty of examples.
I believe authors also maintain a website which has errata discovered in some of their code examples. Maybe inquiring minds can spot them.
The level of math knowledge required is basically of college junior/senior level, although references are made to it being used as graduate level textbook. However, there is really no need to follow all of the proofs and deductions that are offered; this only goes to show the rigor of the approach. The most important takeaway is that the concepts are presented in a manner easily understood, and even more fortunate the cornucopia of code can be run on the computer with one’s own data and models. You be the judge.
Reference book of 386 pages
Financial_Analytics_with_R_Building a Financial Desktop Laboratory
In addition there is also an excellent reference available online showing R’s usefulness as a tool for working with data generally. Most of the topics covered are applicable to financial analysis as well.