Well, @ThomasMitchell is strictly correct. However at the same time, if you have a course, then in the final Bonus Lecture, and only in the final Bonus Lecture, you can do as much promotion as you wish, including links to ebooks of your course, or anything else you wish:
https://support.udemy.com/hc/en-us/articles/229232847-Bonus-Lecture-Rules-and-Guidelines
Having said that, I have not had many sales of my own course as an ebook from this Bonus Lecture, so you should probably expect any revenue from it to be much less than the revenue / enrollments to your course itself. In other words, if we round down the results, Thomas is correct again 😉
Cheers,
William
