Apologies if this should be obvious (there's a related discussion here) But, I've only recently taken the plunge into learning music theory and an instrument. I've been reading up on harmony and voice-leading and then studying arrangements of my favorite hymns and carols to try and see how these ideas are put into practice. However, I'm finding a lot of disconnects...
Everything that I've read concerning voice-leading (and harmony) seems tailored specifically for common practice polyphonic music (as I understand it) and the hymn arrangements seem to be "breaking the rules" all over the place. Of course, hymns seem to be a hybrid of homophony and polyphony. Rhythmically, they're predominantly homophonic. However, they seem to consistently avoid parallel fifths, open fifths, and parallel octaves. However, unisons between inner voices are not uncommon and, specifically, open octaves/fifteenths/etc are everywhere! I run across chords all the time where the root or the fifth appears in three of the four voices (so, not even a chord by some people's narrow definition, since there are only two distinct notes).
Are there any good resources (books, etc.) that deal specifically with the voice leading rules for hymns? I want to understand why things sound good so I can learn how to make interesting arrangements, but applying the basic discussions of voice-leading that I've read so far just looks like "rules" are being broken all over the place. (I understand that I need to look more into functional harmony, but it seems like voice-leading is really what dictates the form of the chords - e.g., why one would double the third when that's supposed to be discouraged, or why an open interval has the root in three voices, etc.)
EDIT To clarify, I'm referring to traditional protestant hymns (1700s-late 1800s is an off-the-cuff time estimate) as opposed to modern worship-band or gospel style songs.