OpenStax
Open Stax: Catherine Schmidt Jones: Duration: Notes Lengths in Written Music
Read everything there is to know about the basics of notes. From their durations, their shapes and their stems, you will learn how notes are used in music.
Music Theory
Music theory.net: Rest Duration
A wonderful resource for teachers and students! Gives examples and explanations of rests and how they affect the sound of a musical piece. Simple illustrations and explanations make this animated site easy to follow.
Music Education
Introduction to Reading Music: Types of Notes
A tutorial for beginners learning to read music.