Clan Boss. Also Challenges, maybe some Missions too, account level progress rewards, daily login rewards.

You cannot farm Epic books in any of the regular dungeons.

Cool, but thats not going to cover remaining 80% of the champs and I think it would take at least half a year of gaming to cover the other 20%. :( 

Do you really need all that 20 champs booked? Some champs you wouldn't use. Some are fine without books. And in half a year you'll get more epic and legendary champs.

Anyway, the best you can do to get more epic books is build a clan boss team that gets the last chest on Nightmare (or even better, Ultra Nightmare) and join a clan that kills it every day. Double NM chests give 1 epic or legendary book every 4 days on average.

Another source of epic books are events and tournaments. Save your resources (gems, energy, silver for artifact event, shards for summon events). Spend them when you know you have enough to get a book. It usually requires quite a lot of points, so you have to skip a couple of events to push one.

I need all good epic champs for faction wars, doom tower, arena counter attack teams.  Getting one book per week from clan boss is nothing but nuisance ( if it drops, I guess ).  Also you need to be in clan, that is able down that difficulty level boss and have champs that can handle that challange. 

I guess, I will die from old age unless I dump lots of cash before I get all the book I need for " only epic champs". 

Only good thing out of it, there is handful of rare champs that are worth booking, everything else is trash.

Credit card is the quickest way.

$$$$ Faction wars 2nd boss, missions, challenges, and other rewards like that. Money is that fastest way if you really want the books. 

Depending on clan boss level, I get quiet a bit but im also getting double UNM chest. Always aim for top chest!