A reference frequently cited is Making and Playing Musical Instruments by Jack Botermanns, Herman Dewitt, and Hans Goddefroy, University of Washington Press, Seattle.

Not a very large book, but it does contain discussion of sound production methods, some information on materials you can use, and very useful dimensions for making specific instruments.

Ten different kinds of flutes (including CleighClay.
About 15 kinds of "idiophones" such as rattles, clappers, "Jew's harp," and "musical saw."
A half dozen styles of drums.
Tin can banjo, African lyre, lute, clog fiddle, and hurdy gurdy.

If "real tools" are acceptable, The Amateur Wind Instrument Maker by Trevor Robinson (no relation), University of Massachusetts Press, has real dimensions and construction details for recorders, flutes, and more complex/modern instruments. Fewer pretty pictures and "primitive" instruments than the above, but very useable information.

I have a listing in my library index for Musical Instrument Design by Hopkin, but can't locate the book for a proper citation. My recollection is that this one deals mostly with instruments children could make - i.e. more "make from junk," although I can't be sure until I find it.

These three are ones I've found cited in more advanced instrument literature. Apparently they are well known, and might well be at your local library. None of the three cost me more than $25, but that was up to 6 years ago.

John