Given the chord notes A-C-E-G#-B, what is the correct chord name assuming the song is in the key of Am? It is an E/Am polychord and the dissonance between the C and the B is the sound I'm seeking, so simply writing this as E/A doesn't really work. I could write it as Cmaj7#5/A, I suppose, but that looks weird. I believe it is an Am(maj7) with an added ninth but I'm not sure exactly how to write this on a chord chart.
This chord is called Am maj9 ("A minor major 9"). On a chart it would often be written just a Am maj7, which describes the basic chord quality. The additional tension (9) would be left at the discretion of the musicians.
Note that major 7th chords are often written using the triangle symbol Δ. This is also true for minor chords with a major seventh (and a major ninth): AmΔ7, or AmΔ9.
The modifier "add 9" is only used if the 7th scale degree is not part of the chord, e.g., Am(add9) being A-C-E-B.