As a fairly recent newbie, I remember the pain and confusion very well. I didn't know then about Pablo's "Quick & Dirty Epub Tutorial" which was just being born (see this thread:
https://www.mobileread.com/forums/sho...d.php?t=130390 ). Now it is a wonderful reference for those starting out.
I wanted to learn about the HTML and CSS first, so I did download some books from the MobileRead library, but because I didn't know what to look for, I made unfortunate choices. The books I looked at had lots of Sigil-created inline code (confusing because the same "class name" was used in different chapters assigning different styles), and were purest gibberish to someone with no html or CSS knowledge. Now that I know what I'm looking at, I would recommend to look at books uploaded by Jellby, SBT, AlexBell, Zelda Pinwheel (these are just off the top of my head, there are many other uploaders doing very nice books), to see code that is concise and often nicely commented to give you an idea of what the styles actually do.
I never did learn how to use Sigil BookView, except for viewing final appearance. I work mostly in CodeView in combination with Notepad++. I think maybe learning in bookview first lets you develop *bad habits* you need to unlearn later as you get more experience.