Here you go:

https://www.w3.org/Graphics/JPEG/itu-t81.pdf

https://www.w3.org/Graphics/JPEG/jfif3.pdf

I'll also link Cristi Cuturicu's "A note about the JPEG decoding algorithm", which is where I started my decoder implementation from, and it was indeed a ton of fun.

http://www.opennet.ru/docs/formats/jpeg.txt