Don't be. The first thing I did when reading your above comment was to do the Google search. I should have scrolled down!
I'm browsing through this, and it's clear to me this is a textbook. In that it easily looks like it could be the textbook of record for college courses. This is an enormous amount of work. Am I correct in assuming it grew out of your lectures notes? What lead you to make this a freely available textbook, instead of going the "normal" route of going through a publisher and getting royalties?
Yes, it is a text. As the linked-to page says, it has been used in hundreds of classes at many schools as well as by thousands of individuals for independent study.
> Am I correct in assuming it grew out of your lectures notes?
No, really I wrote it intentionally not organically. I used Strang's book in a course a couple of times and while that is a very fine book, the students I had in front of me had trouble with it (and anyway I wanted to cover a somewhat different set of topics). I looked around some more but basically I couldn't find a text that fit.
> What lead you to make this a freely available textbook, instead of going the "normal" route of going through a publisher and getting royalties?
I wrote it using LaTeX, on Linux, using emacs. It seemed natural.
I do get some money, from Amazon sales, because it would be stupid to not round the price up. (But in general, everyone tells you that unless you write a very popular text for a very big audience, you are not going to see much money. You need to get your pleasure from the creative accomplishment.)
