The book and software is under the MIT-like license https://people.inf.ethz.ch/wirth/ProjectOberon/license.txt
(It's still polite to ask/let the heirs know what you're doing.)
you're suggesting offering his heirs the opportunity to revoke that license
how can you possibly think this is a good idea? if it became a widespread practice, it would result in a million lawsuits like the sco lawsuit, utterly destroying free software. is that what you want?