Ninja or Tuhanarak, depends on your account. Black Knight is not that bookworthy according to HH website

tuhanarak is S tier according to a list i saw. Ninja A

I'm afraid i dont have a strategically structured account (yet), basically a mangle of what i find 

Tuhanarak is by far the best champion of that list - you'll still want to take Ninja to 6* before her because he's better at campaign farming which is your first priority, but for books I'd probably go with Tuhanarak first. 

I  think  you  should  save  your  books,  until  you  figure  out  your  goals.

Tier  List  are  created  to  give new players  an  idea  of  which  heroes  are  more  useful.

However,  you  should  never  book  a  champion with  out  knowing if  you  can  use  them.

Let  me  give  an  example:

i

Elegaius  is  9  in  Arena.

Elegaius  is  2  in  Clan  Boss

Elegaius is  7.3  in  overall  ranking.

i

Now  lets  say  your  goal  was  to  improve  your  clan  boss  team.

Elegaius would  be  trash  in  Clan  Boss.

However,  Elegaius overall  score  is  7.3.

Elegaius  is  great  hero,  but  she  isn't a  Clan  Boss  hero.

"This  Example" I  am  showing  to  you  is  designed to  get  you  to  understand Tier  List  are  only  information compiled  by  players.

This  information can  be  very  helpful or  it  can  be  very  unhelpful.

The  way  you  use  the  information can  determine if  your  account progresses  forward  or  backwards.

Elegaius  is  great  hero.

I  own  her.

I  use  her.

She  is  amazing  in  Faction  Wars  &  Arena.

She  is  terrible in  Clan  Boss.

I  am  aware  Elegaius is  bad  in  Clan  Boss  so  I  use  different heroes  for  my  Clan  Boss  team.

I  use  Elegaius as  a  Faction  War  /  Arena  specialist.

thx guys, 

i'm surprised how well designed this game actually is. the balance between trying to get you into actually investing vs F2P (yet enjoying) is a challenge for a lot of development firms and they seem to tackle it quite well