Sheet Music Reference & Price Guide from Collector Books, ISBN 0891456481 has been in book stores in my area fairly recently, (about $18 US) and is probably just one of several similar. You might find it at a local library, and might find it interesting to check out whether there are any "special treasures" in your collection. This one does include music from the era you have.

My caveat to my S.O. when she announces that she's found out one of her "collectibles" is really valuable - "The price listed is what one d...d fool with too much money paid for the one he found. That d...d fool already has one, so it's only worth that if you can find another too-rich idiot with bad taste."

Actually, most "collectible" sheet music trades for rather modest prices, and must generally be in "pristine" condition. On the off-chance that you might find something truly "rare," you might consider offering it - or at least an opportunity to copy it - to a nearby museum or library - if one has an interest. It could provide the "entry" to someone who could advise you on preservation of the rest of your collection.

John