How do I view two Books together without merging them?

Put both In view. The trade views read a set of Books, so several can be read at once and taken out again with nothing moved and nothing rewritten.

This is the part that makes Books safe to use. "In view" is a reading instruction, not an edit — it changes which rows you are looking at, never which Book a row belongs to.

So you can hold a Book aside for a month, look at it next to your main record for an afternoon to compare, and take it back out. Nothing about the trades changed.

Contrast that with Bulk operations at the foot of the page — Move trades… and Delete trades…. Those are real edits, and both always tell you exactly how many rows they are about to touch, counted from the same filter the operation will run rather than estimated from what happens to be on screen.