If you only want the names a lot of them will be in the last Dirr manual. From there you might search the web for more recent introductions. If you want descriptive information you should look for everything you can find, in order to form your own picture of each. When you do a survey like this it soon becomes apparent which information is the same concepts being repeated by multiple venues and what bits are something additional. With less widely known or frequently described kinds you may not find a consensus, just have to settle for what seems like the best information.

When you find out who developed and/or introduced a particular cultivar, see if you can find their own description of it. Ideally you would look at the first printed appearance of the plant in the introducer's catalog, this is the horticultural equivalent of an original botanical description of a wild plant. That way you get it from the horse's mouth, before it has been altered by others (should inaccuracies have crept in over time). But such often requires tunneling through collections at university libraries, and in this day of the internet a new garden variety's first appearance might be on a web site, where there may not be any dating to show when it was first described there.

For a species list (with descriptions), eFloras will give you a start:

eFloras Spiraea Key

Wikipedia has a decent start on a nothospecies list:

Wikipedia Spiraea Article

You could also check out ZipcodeZoo for a kind of random mess of a list of possible spiraea names:

ZipcodeZoo's Spiraea Article

Thanks for the great information brandon and bboy. Bboy I work with a young woman that I call Bgirl, just a cute coincident. Do you like the new Dirr book? My birthday is coming up I hoping someone will get it for me.