Nightmare and especially Ultrannightmare clanboss.

Many legendaries work just fine withouot books.

I  get  around  8  to  12  a  month  from  CB.  It  takes a good while  to  get  there, but then it's like a snowball. Just keep at it.

Hoarding and participating in events where they give them out is also decent , but if i learned anything in 50days or so .... legendary tome will be scare till endgame farming , especially once you get all of those from challenge and whatnot!

Those events and tournaments will get a lot easier later, though still can be some work.

That said, getting to NM/UNM CB is the big thing, like the folks above have said. 

Also - and this might be a bit down the road for you - Faction Wars gives out a truckload of legendary books as you work your way through it. You get 10 from getting stars across all Factions, and another 13 in total for finishing off each Factions final stage, too. Though this isn't farming, it is a big influx of books. 

The time you are halfway to finish faction wars, you can 2key UNM...

Heck I 2key UNM before I finished 4th faction.

I get them from clan boss mostly, but they are a big bottleneck for me as well, i have 4 nice lego champs i would quite like to build and books for one of them.

Well, yes. 

The point is once you get to a certain point in the game the roadblock opens up a bit and you do get your hands on quite a few books. Though admittedly, those lego books you get from towards the end of Faction Wars should immediately be going into Lydia.