Books like Basic Music Theory
Jonathan Harnum
Basic Music Theory is 299 pages, about 5 hours 29 minutes at 250 words a minute.
7 books to read next
How long each takes
The Music Kit 6.1 h 
Guitar Basics 53 min 
The Elements of Music 2.3 h 
Learn to Read Music 3.2 h 
Ear-training and Sight-singing Applied to Elementary Musical Theory 3.2 h 
Concise Introduction to Tonal Harmony 8.2 h 
The Complete Musician 12.0 h Bar height shows length. Estimated at 250 words a minute.