One way to promote your well-established open-source textbook is to get a review published in MAA Reviews. A number of reviews of open-source texts have already appeared in that venue: Guichard: Calculus, Beezer: Linear Algebra, Judson: Abstract Algebra, and Hammack: Book of Proof to name a few.
Several of these reviews I solicited by contacting the authors, finding users of their texts and subsequently asking those users to submit reviews. The editor of the MAA Reviews, Fernando Gouvêa, has been supportive of efforts to place reviews of open-source textbooks on the site. If you are a user (not author) of an open-source text, I encourage you to submit a review of the text to MAA Reviews. Here are some instructions on how to contribute.
If you are an author of an open-source text, ask one or more of your users to write a review for MAA Reviews. I wrote a review of Guichard's Calculus text jointly with Victor Reiner. The possibility of several users of the text jointly writing a review exists and, in some ways, could be construed as a stronger review. Of course, best practices dictate that you (the author) probably shouldn't write your own review. :) I think Prof. Gouvêa might catch on to that.