Builders Log and Manual
As a fourth owner, the manual is invaluable in seeing the mindset and context of the builder of the plane when I need to repair it. The build log can be helpful in this regard if it's very detailed, but otherwise is more a nostalgia thing. I have the "build log" of my plane, built in 1996, and it's neat seeing fading 35mm pictures of the builder with the plane in his garage, next to some sort of early 90's car. It's not helpful from a detail standpoint, but I like looking at them just to know someone put a lot of work into the plane I am still flying, long after they are dead.
You mention a will, so I am guessing the thought is "I would like my family to have something to remember all the time and love I spent on building an airplane", with pictures and comments of the plane. If it's not detailed enough to help the next owner, I would just keep it. If it's super detailed, and would help the next owner, I would copy it or scan it, and keep the original.
The manual (I assume you mean build plans etc) in my opinion should definitely go with the plane. They can buy new plans from Van's, but those are always the latest, and may not include "margin notes" you made while building that would help them out". Same thing though, if they are sentimental, get them duplicated and send those.
Legally, I don't think you need to send either one with the plane, just the AROW docs basically.