Prince Malchezar is the 2nd Neutral Demon minion, the first is Illidan.
Seems like an auto-include, immediately increases your deck size to 35 cards, but granted this means you could have some dead draws which fuck you up. For a Control/Mid-Range deck it's great but for Tempo/Aggro it might not be desirable.
5 mana 5/6 is really nice stats, I hate to see the 5 slot keep getting inflated. 5 Health was a leap now it seems 6 health is the new mark.
Will be great for Reno decks:
It will NOT add duplicate legendaries into your deck. (if you have Reno in your deck, it will not add another Reno.)
It will obey deck-building rules. If you're a mage, it can only add Mage and Neutral Legendaries.
Protect the King is interesting. Warrior only, can serve a few purposes. In Control it functions as a counter to zoo, but otherwise pretty useless. Where this card could shine is in Aggro/Tempo/Fast decks. You can combo this with Frothing and a whirlwind to get huge Frothings. Can also combo with Cult Master and whirlwind. Might be too inconsistent as you're relying on enemy to have a board, but even 1-2 minions to get a few extra cards or frothing buffs could be worth it. Right now I'll say it's a niche card that sees little play.
Edit: Forgot the potential for Bolster Warrior. Another Warrior deck inbound?
Book Wyrm seems too conditional for the cost. You need to hold a dragon and enemy needs to have a 3 attack or less, that's 2 conditions needing to be met and it also costs 6 mana. Is this the first 2-condition minion? Seems really expensive for such a small body on 6 mana. I just cant see it. Blackwing Corrupter has a better body, cheaper, and kills most 3 attack minions plus can damage higher health and even face. This card just sucks imo.
Moroes is really weak to Warrior but infinite stealth is very strong. I can actually see this in Yogg Druid and maybe an aggro paladin deck. Might be a card we'll all come to hate. Won't see play in most decks.
Edit:
Malchezar's Imp is interesting actually. Maybe fits in zoo or even a super-fast aggro warlock deck might come into play. Succubus, Darkshire Librarian and Tiny Knight of Evil will all work well if this is on the board, you won't drop your hand, you'll discard some cards but the immediate card-draw keeps your engine running. Interesting card to allow for a new type of warlock deck. Might not be viable or consistent.
Babbling Book seems bad but I've been wrong before. Mage 1-drops already: Mana Wyrm, Mirror Images, Arcane Missles, Arcane Blast. Dropping a 1/1 for a random spell just doesn't seem worth and bloats the 1-drops in your list. I think it's bad.