Absolutely not. I know you got away with this once on meta, and TBH I don't think that was appropriate either, but let's just leave that at that.
Here's the problem: So you wrote a book/piece of software/designed a HAT/a case/a GUI theme, etc. etc. -- something that in someway relates to the Raspberry Pi and that you would like to distribute and promote online.
It doesn't matter whether it is for money or not, because if we say, "Sure free stuff is okay," that is arbitrary and prejudicial. It also requires we determine the truth of such claims and invites people to come up with schemes to circumvent how we determine whether it is true or not.
So, now we have a very broad category of things applicable for sale or free distribution:
- If books, why not blogs?
- If blogs, why not any kind of online anything?
- If books, blogs, and online material, why not software?
- If books, blogs, and software, including things people must pay for, why not pi related hardware products? Aren't things like HATs, power switch boards, etc. etc. valuable and interesting to the community too?
Great! Now we are Craig's list for the pi. Why not? Because:
At some point I strongly suspect SE management will intervene. This is not a free advertising site.
Part of what I imagine would justify #1 is this then may come to seriously interfere with the primary purpose of the site, which is people asking technical questions and getting answers. It is not here for you to talk about your projects and so on. If you want to do that in chat, terrific, there's a good chance other people may be interested.
But not twisted into a Q&A format. What is then to stop Adafruit and Sparkfun from creating accounts and then making announcements with links to products and online material? How about Packt or O'Reilly or Leanpub? Or anyone? While I doubt the aforementioned organizations would, I guarantee that there will be others that do. Do we start to pick and choose who counts as "okay" and who is "out-of-bounds" or do we simply say:
No Advertising. Period.
There is already a sort of exception to this which is commonly used here and elsewhere on SE -- if you have a piece of software or online material that you think can legitimately be used in an answer, that's fine, although ideally you should acknowledge you are the author.
Creating Q&As explicitly for that purpose, however, is a grey area.
Also, you can promote yourself and your wares as much as you want in your profile. But no "signature" lines in posts.