Quite a few, but personally I’ve never found any of them as enlightening as the last thing I read, which isn’t a book but a dissertation that is also available online: Lambros Dragatakis, The Organology of the Tambouras in Greek and International Bibliography, University of Ioannina, Department of Music Studies, Arta, 2019.
You’ll find roughly anything written up to 2019 in the bibliography there. You’ll also find a critical assessment of each source, because don’t think it’s enough to find something written somewhere saying “the tambouras was like this” for it to be true as well; things are much more complicated than that.
Naturally, the very well-known book here on the forum by St. Kourousis, From the Tambouras to the Bouzouki (see here), second edition, 2 volumes, Athens, 2022 and 2023, also discusses the tambouras.
I also usually refer people to a text of my own, “The Tambouras Instruments”. It’s a little old, from 2009, and since then many of the things it says have been presented in greater detail in newer and better texts by others. I don’t think they’ve been disproved, though — as far as I remember, at any rate.
More generally, if you patiently search the forum using the words tambouras and tambourades, you’ll find a lot, though not all of it will be useful: some posts certainly merely express questions; in others, someone may be looking for, say, a student tambouras; in yet others, we may be discussing things in the dark that none of the participants knows enough about. But with some careful sifting, among all the rest you’ll also find the sources you’re looking for.