Here is a good place to start, they have a forum for each of the major units of the USAAF:

http://forum.armyairforces.com/491st-BG-Metfield-North-Pickenham-f51.aspx

The 491st BG(H) have a website: http://www.491st.org/

If you are looking for in depth research about a specific person/mission/aircraft there is an excellent researcher who I have used numerous times, he's not free but he's usually pretty quick and quite good at finding what seem like needles in haystacks:

http://etsproductionsllc.com/Home.html

I hope you and your friend find what you are looking for, these men deserve more attention and appreciation for their sacrifices!

Google books churned out this result https://www.google.co.uk/search?tbm=bks&hl=en&q=b-24+491st+855th