Nothing says you have to write the whole book start to finish

Or even a whole chapter start to finish

You'll find, as you go, that your outline/plan will morph and shift as you go, that some part(s) will be better elsewhere, etc

Don't be afraid to cut-out content, move it, reword it, etc

Some things have to be done in order (you can't throw a roof up in the air and build the walls under it so it lands on them) ... but many things are not sequence-dependent during creation

Keep this in mind especially if you're writing something of the "technical" or "reference" variety - your audience may (but probably won't) read it all in the order you wrote it. They could easily find half of chapter two to be where they start, move to the end of chapter nine, and back to the middle of four

This is so clarifying! Thanks so much!!