Thank you very much, @cderv! I followed your suggestion and it worked. I've just had to put inside an iframe to avoid css conflict since I am using {shinymaterial} framework.
shiny::tags$iframe(src = ("book/index.html"), frameborder="0", style="overflow:hidden;height:100vh;width:100%")
Sort of.... Could say that it was one of the reasons. My app do other stuff besides showing the bookdown.
But I'd like to hear more about it! Is there any convenient setup for login purposes that am I missing?