I’m currently doing R&D on a physical instrument/sound generator, and my go-to practical reference book has always been this one:

Musical Instrument Design – Practical Information for Instrument Making
by Bart Hopkins

Another book I found recently, ‘rented’ for free as an ebook via Archive.org

Vibrations : Making unorthodox musical instruments
by Sawyer, David

This one looks interesting:
Acoustics of Wood
by Voichita Bucur
but the purchase price is rather prohibitive at $379.99 for a paperback copy….

Another free to rent ebook from Archive.org

Tone : a study in musical acoustics​
by Levarie, Siegmund, 1914-2010

Please comment if you know of any other books or resources?

UPDATE:
Sound Designs: a handbook of musical instrument building
by Reinhold Banek & Jon Scoville

UPDATE:
Sound Sculpture, ed. John Grayson
Download [PDF, 24mb] at ubuweb
A collection of essays by artists surveying the techniques; applications; and future directions of sound sculpture.

UPDATE via comments, thanks Dan:

There are a couple of other, smaller, Bart Hopkin books I’ve also found helpful. Built a few things over the years. Not sure which book I found it in, but the last one I built and sampled was this waterfall: https://barthopkin.com/instrumentarium/waterfall/

Back issues of his magazine “Experimental Musical Instruments” are brilliant too:
https://archive.org/details/emi_archive/0-START-HERE/mode/1up
Table of Contents PDF of all issues is text searchable