Play E9 one string at a time and you hear E, G#, B, D and F#. E is the root, G# the major third, B the fifth, D the minor seventh and F# the ninth. Said aloud, its notes are E, G sharp, B, D and F sharp.
String by string, low to high, it plays E, B, D, G#, B and F#. B appears twice in the main shape, and it is the chord's fifth. It runs from E on string 6 up to F# on string 1.
The open strings add notes to E9 too. String 6 gives E, string 4 gives D and string 2 gives B. Your hand covers two frets, from 1 to 2. There is one more position, starting at fret 7.
Change only string 1 in E9 and you have E7. Inside E9 sits a complete Bm chord, made of B, D and F#. Moving to E, you keep two fingers down.
E9 belongs to only one key, A major, where it is the V9.