At the moment google are phasing in a new tab system for their gmail accounts. This splits up promotional emails (which google identifies) into a separate tab.
You can opt back out (by going into settings and unticking all tabs) which puts it back to how it used to be.
Of course, most won't bother. Included in the new promotional tab are some of the big names in eBook advertising - KND, BookBub, KFD, POI, ENT etc.
Having a whole 'I'm trying to sell you something' inbox essentially does one thing: it makes it very easy to ignore promotional emails. It's a marketing email ghetto.
These are "Bacon" emails. Not spam, as you did sign up for them, but not stuff you really need either. We've all had them - and largely we gloss over them. This new system makes that even easier.
I suspect google uses the number of recipients / # emails sent by each email header to identify promotional emails (as well as contextual content analysis) and it looks like they're doing a very good job of it. My 'Primary' tab now contains mostly emails I actually want, and I can't see any emails that have gone AWOL - yet.
For authors, this probably means email lists, and email blast advertising websites like BookBub et al will become a little less effective. Gmail is huge in the consumer market, and the filter will add an extra click to get people to the promotional emails (and one click means a huge drop off in activity in internet terms) - in reverse, this means easier = more buys, see amazon one click as a good example).
It's impossible to quantify the effect right now, but it'll be worth keeping an eye on the efficacy of these promo emails over the coming months. Social media elements - particularly big facebook pages and twitter followings, will lessen the impact, but it may well be measurable.