I'm searching for a trigonometry book that explains everything in depth. I mean, in completely depth, explaining every single thing about trigonometry (a.k.a. explains from where comes each trigonometric ratio, how I can prove that it works...).
I found a pdf of Gelfand's book and read first pages of definition of sine, but it seemed to me that he doesn't explain everything. Maybe, he just don't want to start briskly and will explain more in depth in the following chapters.
Any suggestion? Do you think Gelfand actually can satisfy me?