I just got a letter saying that ACA believes they have money for me. Notably large sum. They want some personal information. They claim to be associated with musicians, authors, illustrators, and so forth. A search shows them to be possibly legit, but I am asking if anyone else has ever heard of them or had any experience with them.

1- how did they find your address? was it an email?
2- http://www.authorscoalition.org/ if it's these guys, don't you have to join first?
3- why do they need your details? obviously they can send you a letter.. this means they have got your address.. they should send it to you without you doing much..

I personally never tend to believe people who tell me they will give me money out of nowhere.. it's usually a scam.. I got a handful letters in the past saying "You won the european lottery! All you need to do is: provide this and provide that"

I'm still skeptical of anything that claims to have a "large sum of money" for something I never heard of.

http://www.luminous-landscape.com/forum/index.php?topic=65660.0

But these testimonials could be shill accounts?

I couldn't find anything negative, just people who said they got money. I still don't understand. Also, make sure it's from them and the address and website address match http://www.authorscoalition.org/index.html

It is mentioned on Wikipedia:

http://en.wikipedia.org/wiki/American_Society_of_Journalists_and_Authors

ASJA is a member of the Council of National Journalism Organizations  and of the Authors Coalition of America

I believe it is legit.