Announce away. If you've written and published a book then it's definitely not crass to announce it in a story or put a link at the end of each story for people to go and download it.

Don't you worry about buying me a coffee. The fact you're here and writing comments pushes your read-time up, so engaging in the comments section is just as good.

If you copy and paste the URL of an article on Medium into one of your articles and then hit enter, you'll get a little box link to one of your articles. Annoyingly, I didn't do it on this story, but I did on my most recent one. https://medium.com/lucid-nightmare/creative-people-are-a-drain-on-society-a-london-café-showdown-8918fe9ed9cf

Also, if you highlight some text on one of your articles as you write, you'll get a little menu pop up to format your text. On that menu is a button which looks like two-chains being linked together. Click that and if you paste a URL into that space, it'll turn your text into a link for somewhere else on the internet.

Hope that helped :o)