If you would like to pursue ebooks, I think you're better off taking a look at self-publishing. Kindle, Nook, Kobo...all of these places allow you to upload your material for sale. Yes, the price points are cheaper than traditional PDF ebooks, but you can get more sales.
How much you can earn is just like anything else. Selecting good keywords to use in the title, writing excellent blurbs, and posting good covers will help you drive sales. Some niches are more saturated than others.
But I do believe posting ebook guides in this fashion is a more stable way to earn some extra money. It just takes patience and persistence. Your books don't have to be long treatises. I've sold quite a few 5-10K word guides in my niches and never had a bad review or returns. I've been doing this for about a year now.
The good news is that Amazon shows no sign of screwing you overnight the way a Google algorithmic change can. Changes to their search functions are incremental, and often increase discountability of your books on their market. Uploading to other places like Barnes & Noble is a good opportunity to diversify too.