I agree with this.
But that's the thing, you can get really good at international taxation, but of course that for complex local structures you will need someone who knows the local stuff.
International taxation is somehow akin to general medicine. You can get really good at knowing what's going on with a patient and what will probably be good for his/her symptoms, and you can even take care of them if simple/generic enough, but if they are too complex, require surgery or other treatments, you'll refer the patient to the appropriate specialist.
But by doing so you are still a really good primary care doctor, you simply cannot do everything yourself.
And back to the initial premise: how do you get knowledge about international taxation? There is no way around it other than reading and reading. There is no shortcut.
But the good news is that everything is online in tax treaties, summaries, OECD documents, IRS website, etc. Even concepts such as residence, source, etc. are all in there and those definitions vary by country (or are applied differenty in each) as well.
For instance, the books that JohnnyDoe showed, although they look intimidating, are just overviews of around 100 tax jurisdictions, very similar to what one can find in PwC or Deloitte summaries.
Ok, I see your point.