You might want to check this declass from the FBI Vault about a group called the Finders:
https://vault.fbi.gov/the-finders/the-finders-part-01-of-01/view
This info dropped last Friday and does point out the existence of the tunnels under McMartin. We now know the kids were telling the truth about the tunnels and more. There was a lot of CIA involvement in this operation as well from this drop. Well worth checking out.
I did check out this book you posted and tried to look up what more about the authors. The only thing I could find is one of them helped co-write a movie called "Cannes Man" and one of the guest stars in the movie was Harvey Weinstein.
CGato
Without having yet read your link, I will point out that we have very few basements in Cali, let alone tunnels under houses in local neighborhoods. Manhattan Beach is right on the ocean and would be an odd place to put a tunnel, the properties are not on large lots at all.
Thank you for combing through the “Finders” and keeping us in the know. I downloaded it. But like so many other things, I have not had time to really read it.