I am a big fan of roda and especially sequel. Both libraries are masterfully developed and maintained by Jeremy Evans. While roda is very nice and offers some features like routing tree, I could live without it, padrino and rails are good too. Value of roda is in amazing stability of the library and great documentation and support from Jeremy. But for me it's the sequel that is irreplaceable. Sequel offers flexibility and code efficiency I haven't seen in any other DB query library. You'll have to pry sequel from my cold dead hands.

I am currently working on a small project that has to use some python libraries because they are just not available in ruby (CAD application). While I can use flask for routing, it's the DB query lib that makes me split it in two, roda+sequel for all front/back end, and passing json only to second part (python flask) that will generate required objects.

Good luck with your book.