I am way behind updating this blog, including mentioning the LibreOffice 6.4 user guides that have been published in 2020. These include Getting Started Guide, Writer Guide, Base Guide, Calc Guide, Draw Guide, and Math Guide.
Two volumes of the v7.x series of books have been published recently: Math Guide and Calc Guide.
You can download free PDFs of these books or buy printed copies of the v6.4 volumes (v7.0 print copies may be available later). See this page for links.