Mobile first

Mobile First, by Luke Wroblewski

Web standards

Designing With Web Standards 3rd Edition, by Jeffrey Zeldman

Adaptive web design

Adaptive Web Design: Crafting Rich Experiences With Progressive Enhancement (Voices That Matter) 2nd Edition, by Aaron Gustafson

Progressive Enhancement

The Uncertain Web: Web Development in a Changing Landscape 1st Edition, by Rob Larsen

Designing With Progressive Enhancement: Building the Web That Works for Everyone 1st Edition by Todd Parker, Patty Toland, Scott Jehl, Maggie Costello Wachs ⭐

Responsive Web Design, by Ethan Marcotte

Accessibility

Accessibility Handbook: Making 508 Compliant Websites, by Katie Cunningham

Universal Design for Web Applications: Web Applications That Reach Everyone 1st Edition, by Wendy Chisholm

Site performance

Designing for Performance: Weighing Aesthetics and Speed 1st Edition, by Lara Callender Hogan

High Performance Mobile Web: Best Practices for Optimizing Mobile Web Apps 1st Edition, by Maximiliano Firtman

Git

ProGit