The first thing to know is that Jupyter Books and “Jupyter Book support” (in Azure Data Studio) are slightly different concepts. Jupyter Books let you build web-based collections of Jupyter notebooks. Jupyter Books support allows you to build collections of Jupyter notebooks on your local computer or network (ie. not web-based). Additionally, all of the standards and functionality of the online Jupyter Books may not be fully supported/implemented in Azure Data Studio.
Click through for what this means as well as what the March 2020 release brought us.