The Little Book of Deep Learning

From the website:

This book is a short introduction to deep learning for readers with a STEM background, originally designed to be read on a phone screen. This is the v1.2 updated on May 19th, 2024.

It is distributed under a non-commercial Creative Commons license and was downloaded 600,000 times in a bit more than a year.

Interestingly, you can download it as a phone-formatted pdf

I own the physical book, and I highly recommend it, it was a nice start for me (software dev) into the field.