You will get plenty of rares books eventually and Kael is well worth your books. So I would say definately use the rare books you have on him.

As for epic books and legendary books definaely hold on to them. Never use them until your champions is level 60. Just to be sure you dont get a better champion before you can make use of the books.

Also seek advice before using your epic/legendary books.

It is fine to waste your mystery shards. In the beginning their best use will be to pull champions for food, maybe even just to get a few point in summoning events for some extra energy. I recommend buying every mystery shard from the markets they are useful even end-game for some extra points in events.

My leveling strategy is to even level up your level ones and use sparring pit for the last level to save abit of energy instead of overlevelling. It is just abit more energy efficient. Some feed their level ones and only level from level 2. It is  more energy efficient but in my experience it will drain your reserve of mystery shards in early game where you level alot.

Edit: and I didnt read the last page, so didnt see you already were answered - well hopefully you will get some value from my post aswell

I had over 100 mystery shards and am down to about 20 now.  I have been using them to feed the farming process.  When I want to get junk, I get lucky and hit some uncommons with value and even epics.  lol

Is there a limit to how many mystery shards I can have on hand? 

I started on the hard level of campaign.  But they cost more energy and give less XP.  I would like to use hard level to get better gear but cant waste energy. 

Mystery shards are not too much of value but you can sometimes get good rares from them. I would wait until a summoning tournament or event than open them all, but otherwise it doesn’t matter when you open those. There is also no limit so they can keep stockpiling up.