The Install option as a PWA is not available for Hooktheory I in Windows 10.
I was able to install Hooktheory II
Can you please check?
Can confirm this issue, not seeing the option either in Book I. Installing Book II is possible 
The Install option as a PWA is not available for Hooktheory I in Windows 10.
I was able to install Hooktheory II
Can you please check?
Can confirm this issue, not seeing the option either in Book I. Installing Book II is possible