Basic skills:
The Ultimate Handbook for Beginners
Ultimate Frisbee Skills & Strategies

Intermediate skills:
The Ultimate Handbook

Advanced skills:
The Huddle
Understanding Ultimate

Throwing exercises:
The Wiggins Zen Throwing Routine
Kung Fu Throwing Routine

Drills:
USA Ultimate Skill Development
The Ultimate Skills and Drills

Video:
Throwing & Handling Skills, Field Strategy
Brodie Smith Youtube channel (search for the ‘how to’ videos on his channel)

eBooks:
Rules Ultimate Frisbee 2017 English

Forum:
Ultitalk