Advice for dealing with possibly thousands of inherited books

So, my MIL was an art historian and had a collection of thousands of books that my husband has inherited. I know we shouldn't deal with books stores and dealers and just catalogue them and sell them ourselves. Most of what I am dealing with are reference books, not rare first editions of fiction. I am looking for advice on software and what not. Do I just make an excel spreadsheet or do I get something like Neatoscan? My husband used to work for Goodwill and has some familiarity with their bookselling operation. I wonder if any organizations would want them as donations?

I'm just looking at this mountain wondering where I start. It is two mountains actually, because it is spread between two houses in different states. I don't want to throw them all away if there is something that someone would want. It is hard to even estimate how many there are. Basically, she has built in book cases on at least one wall of every room in both houses and they are FULL. There are tons of nice coffee table books of every artist you can imagine. I know basically which room is which period of art/culture, but she didn't catalogue them. If the zombie apocalypse happens, I could teach humanities! I even have her slide projector and slides of every important piece of art in history!