Practice sight reading Bach chorales, and you'll never have a problem reading "stacked notes" again As for other advice, study a bit of theory and become fluent with chords in every key and you'll begin to identify patterns subconsciously which will make sightreading (life!) much easier.
Whoa. Is it just me, or do you need to have a finger span of a tenth to play some of these? I'm looking at the second chorale in the Amazon book preview, and the middle voice (starting on middle C) is between two notes over 2 and a half octaves apart. Would you just drop a note for that or something?