"The only medium that is still inviolate is books."

Mostly. I actually have some very old books lying around - with advertisement for various, non existent products in them. But gladly I have not seen any open advertisment in any modern book.

Not so inviolate. There are a number of companies (usually high end designer brands; think Prada and jewelry) that commission authors to make books featuring their products. This has been going on for years, but I don't know the efficacy. Then again, no one really knows the efficacy of any kind of advertising.

In a perfect world, the purpose might be something like, to inform us about things we would already want if only we knew about them.

I'm a big critic of marketing, but marketing is supposed to do a lot of stuff. Perhaps persuasion of the form you have experienced is the worst they could do. First of all it should understand a market in order to help build a fitting product, and afterwards it should educate people that might stand to benefit from it, about the existence of said product and the benefits.

Somewhere along the line I think marketing people found themselves singing to the choir of intrusive ads, just as devs find ourselves singing to the choir of Agile (with a capital A) and whatever else might fancy those on top.

Advertising used to be benign things like alerting farmers to an improved plowing technology in a trade magazine. Then companies realized they could get away with selling an unneeded/worse/disposable product if they just outright lied about it, and took advantage of the goodwill the public had towards advertisements to reap profit.