Preview Panel sometimes overlays Code View, so you have to grab the bar at the top of it and tertivate it. After an agonizing while, you'll hit it just right and pop! you will have three panels in a row: Book Browser, Code View, and Preview Panel. Now there's no reason ever to use Book View, because you've got a permanent WYSIWYG display sitting there on the right.
(Well, you can't test the TOC in Preview Panel, so you might want to call up Book View for that.)
Other stuff can creep in. I often get a few faux hyperlinks in a book file, probably created by Word somehow. And recently I discovered that a book (maybe all my books?) was full of invisible soft hyphens, so I had to make them visible (­
in order to delete them.
And yes, rarely, Sigil will create a new style if you are pushing inline styling too far; it prefers that there be a class for it, so creates one at the top of that particular file and will repeat it every time you split off a new file. You can either leave them be or add it to your style sheet.