Most Disney records sell for just a couple of dollars because the music is still available on CDs. And with all of the work Disney has been putting into their CD's for the past 10 years or so, they're much better than the records ever were.
Yours is an exception because of the book and it's vintage. It's from the era of the original release of Dumbo and is obviously filled with great images. I'd value it at around $30-40.00 retail. If you added your wife's mother's notebook to the mix, it could be worth quite a bit more - especially if the notebook referred to the specific Disney movies she was working on. In any case, this is a very nice Disney collectible. Good luck!