I think I saw these on /r/askculinary which is actually a pleasant little community. These books aren't specifically focused on spices per se. But over multiple threads asking similar questions (ie, "theory" of cooking) these were the ones consistently praised:
all on gen.lib.rus.ec
To be clear I have not read any of them but I will one day.
And I haven't seen this one recommended there but I have enjoyed Segnit's The Flavor Thesaurus.
Is that place worth going to? I've heard about it but I also have gotten the impression there's a lot of nazbols there.
Yeahhh! Thank you!