Stamp Identification Book/App/Website/Software

My grandpa's collection includes a number of stamps that are apparently difficult to identify. At this point I don't care about values (I can look those up online once I know what to look for). What I want is a resource with images for comparison, with enough info to help with distinguishing the similar stamps.

If I could only invest in ONE resource, what should that be?

If you say Scott's Catalogue, I don't think I need the most current version. From what I can see after looking up the stamps I've gone through so far, after the 1930's, it looks like they stopped using the same images for several issues, and grandpa appears to have stopped collecting sometime during the early 80's. Nor do I need all volumes--at least for now, I'm focused on US only. Does anyone know where I might find an older US catalogue for cheap?
Thanks!

I'd search ebay, visit my local library, or contact the local stamp club if one exists. There should be numerous inexpensive copies available.

Good luck in your search.

Your local library probably has the current Scott Catalogues. ebay's publications listings usually has reasonably priced catalogues.
You could also try a few websites. The 1847 USA site has a lot of information that may help.

http://www.stampsmarter.com/1847_landing.html

That website looks like an awesome resource! Thanks!

Be aware that in order to properly identify many US stamps (and eventually to assign a value to them) you will need information not contained in the image itself, such as perforation gauge, paper type, watermark, etc., and you will need tools to distinguish among these variables.

oh lord, did someone say Library?

I use this easy one volume cat. $24.95. 2014 year and IMO any thing after 1980 was mas produced--

You could check the library for some catalogues, or ask in a stamp club
Your local stamp dealer might be able to help you

Most public libraries will have a reference copy of the current year and keep a year or two of older ones for circulation.

They also may sell off old copies in an annual used book sale, on a "for sale" shelf, etc.

You can also pick up cheap used copies on places like Amazon, Alibris, half.com, etc.