What is the best resource to assess RR book collections, if any?
"Generic" books or coffee table books are not worth hardly anything...The Central Pacific and Southern Pacific by Lucius Beebe for example. It's an 'old' book but the contents are generic and rather broad. Tony Thompson's Southern Pacific Freight Cars and Pacific Fruit Express are great books and have been reprinted again but before they were reprinted they were pulling prices WAY above their MSRP (you can still find PFE for sale where they haven't bothered to see that it is in reprint for $390+). Some books that are really well done books won't however, be reprinted (publisher and/or author have passed on). Cab Forward by Robert Church is one example (selling for over $200).
My books are going to the California Railroad Museum. I'm sure that they have them all already, but they can resell them and the money will help the museum. Same thing (but slightly different for my N scale. It will go to the train club (Sacramento Valley NTrak) to either be sold to help the club or (even better) given to kids who are interested in trains but can't afford buying their own.