"The Letter Z" by Marie Sexton

This story takes place after "A to Z" with partners Zach and Angelo headed to Las Vegas for a short vacation with their best friends Jared and Matt, who are also partners.  Unfortunately, they run into Zach's ex, Jonathan, who has always regretted letting him go and wants to get back together.  This sets off a very jealous and insecure Angelo.  Jared and Matt try to help, but they soon learn that they've got to let their friends find their own HEA.  This was a soapy, emotional novella that defined Zach and Angelo's relationship.  99 pages (Kindle edition).