Mo, it's out of print. Published in 1984, it remains the only decent reference of M77s. Occasionally copies surface on ebay. I think it is called "Ruger M77 Rifles", by Ronnie Burke. There have been a few romors about that he may be planning an update. It is long past needed.
"Ruger and His Guns", by Wilson is also pretty good, tho not as detailed as Burke. It's regularly seen on ebay as well as Amazon.com.
Chad Hiddleson's "Reference of Ruger Firearms" is good for deciphering models and quantities of rifles made. He is on this forum as Chet15. PM him and he'll sell you a subscription to Red Eagle News Exchange (RENE) the definitive monthly publication of "all things Ruger". A copy of the "Reference", I believe, comes with the subscription. Don't leave home without it.