I've been casually researching books on viruses that I could find free pdfs for and searching the term "mask" in them (checking of course that they were books related to airborne flu or coronaviruses, and not computer viruses) and quite frankly... most of these books don't even contain the word "mask" anywhere in their text, or at most just have a vague recommendation to use them in a laboratory setting with no mention of their actual effectiveness.

I did find out that coronaviruses are between 200-400 nanometers big (for reference a human hair has a width of 80,000 to 100,000 nanometers and most cloth masks can't even stop hairs), the first virus was photographed with an electron microscope in the 1940s, polio was photographed in the 1950s, and that use of masks in hospitals became common to protect against bacterial infections in the 1920s and even then they recommended that they replace masks frequently, and that mask mandates during the spanish flu were basically ineffective.

Now I don't have access to current medical textbooks, but if masks are so effective and proven you'd think that any book about airborne viruses would mention mask use and how effective they were... unless masks were not effective against viruses.

For example: Methods in Molecular Biology 1282, Coronaviruses Methods and Protocols (springer protocols) a textbook on coronavirus copyrighted for 2015 (299 pages) Has the word "mask" once, but it does not refer to medical masks, but rather "masked" in how the virus infiltrates a cell. It has a fairly long section on "prevention" but it doesn't mention masks at all.