Is there some special function making it easier to split in book view?? I just use search and replace to search on whatever code is used to define the beginning of a section (chapter) and replace it with the new page command.
search: <h2> or <p style="page-break-before:always"> (or whatever)
replace: <hr class="sigil_split_marker" />\n<h2>
Followed by F6. This splits them all in a couple seconds. If I find a one-off page that was coded differently as I'm cleaning, then there is a nice little button at the top of the screen to "split at cursor".
Typing "<hr class="sigil_split_marker" />" once every book might be considered a lot of work, so I made it a "clip"...so it ends up being a simple click of the mouse. I have several clips for shortcuts that I use constantly: (.*?), <strong>\1</strong>, <em>\1</em>, <div class="">\1</div>, <span class="">\1</span>, <blockquote class="">\1</bockquote>, etc.
I previously used BV for my final look to see how it would appear in a larger screen but, just yesterday, I found out that you can double-click the header of the preview pane and it toggles from docked to not-docked. If your not-docked window is sized to full screen, then it is even better than BV! If you are lucky enough to have a second monitor, like Ducky mentioned, you can have the preview pane un-docked to the second screen.