Engaging with the natural notes of the (French) horn.
Currently there are educational videos as well as videos for playing along with. Duets, trios etc minus a part so that the experience of playing just intonation intervals can be somewhat 'live'. The first in the series invites you to join me in a duet. Use the harmonics of the horn to achieve simple just intonation in a few standard chords in C major (concert F). Fingerings are suggested and the notation shows the major 3rds that should be narrower than standard 12-tone equal tempered intervals. Notation uses the Extended Helmholts-Ellis Just Intonation system devised by Marc Sabat & Wolfgang von Schweinitz (see their detailed explanation at https://marsbat.space/pdfs/notation.pdf). The video is embedded below for easy access.
There will be a book published at some time in the future.
Some of the videos below are found in the playlist Michael's Microtonal Method. They are for advanced players wishing to expand their knowledge, experience and skills.
Overview - embedded below
Episode One (3rd harmonic) - https://youtu.be/Ur-WWNJARL0
Episode Two (5th harmonic) - https://youtu.be/Yz9anwRH_1M
Episode Three (7th harmonic) - https://youtu.be/YDjYnxAs6Ck
Episode Four (11th harmonic) - https://youtu.be/Jp1qaHl5b0g
Episode Five (13th harmonic) - https://youtu.be/PLnf0lW3XOs.
7th harmonic - https://youtu.be/TPmVYGvXJ2s
11th harmonic - https://youtu.be/0KHtVSxlXI0
13th harmonic - https://youtu.be/Yb1ugIlY6v0