Is it ethical to refer to opening book for the first few moves of a correspondence game? I am still learning chess (been playing ~6 months steadily now and am at ~1300) and have recently been enjoying correspondence chess with a 3 day time control. I find it gives me time to really analyze each move without the pressure of time that I get with blitz games. 

I have been trying to learn new openings in detail, mostly using opening explorer in lichess or chess.com. As most of you probably know, it breaks down the most popular moves in the opening from games played and gives the % outcome of white wins/black wins/draw for each move. This is an amazing resource for someone like myself who doesn't have lines memorized yet, but is it unethical to refer to these resources during a daily game?

I would never refer to an engine for a current game and the opening explorer is only showing the data for games that have been played, not evaluating positions, but I still wonder if it would be wrong to use. 

What is the general consensus of the online chess community on this?
Yes.

ICCF: Says nothing about it, or assistance of any kind, and you can even use computers legally. Here are their rules.

USCF: "3. You may consult chess books and periodicals but not other players." Here are their rules.

With regards to the ICCF rules, my guess is that they just decided it was too hard to police computer used, so they just allow it. Books have always been permitted.