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As Trevor said, you can enter the stuff in any order, but I would suggest that you first think how you can enter the data and be sure that you haven't missed anyone. That might mean that you start with page one and go through, making a note elsewhere of where you're up to. Somehow I find that sequence, front to back, more natural. Others may think differently.
Don't forget to record the source of your information both in itself (probably one source record for the entire book) and against each person and fact that you enter. That's probably quite easy to do - just set FH to repeat the "citation" for the book, each time you enter or update something. I'd advise that you don't omit that step, because sooner or later, you'll be adding someone in because you know that they exist and if you have no sources recorded, you won't know whether it's from the book or not, later on.
Feel free to set up something, enter a few to get the hang of it, and then delete it all once you know what you're doing.
When you come to check, that's when I'd definitely start with what you know - yourself.