The definitive page for any of the Active Calculus texts is https://activecalculus.org and you can learn much more about the three texts at the learn more links above.

All three texts are free and open-source and are a response to the problem of textbook companies selling overpriced books to students. I won't bother to link to it, but there's now a 9th edition of Stewart's Calculus that's for sale on Amazon for $282. Calculus belongs to humankind, and anyone who wants to learn the subject should be able to read good books about it for free. Or buy a print copy for under $25. And if Active Calculus doesn't suit your tastes, there are certainly other good choices.

Active Calculus - single variable was first made publicly available in January 2013. The PDF version has been downloaded over 200,000 times since August 2014. The text is free and open-source; the source code can be found on GitHub. The single variable text was the first of the three and recently turned 10 years old. More history of the other three texts is shared in that link.