This is a slow/controllish midrange deck that you can build up the handsize and use it to your advantage, and on top of that you have a good dragon synergy. Very fun deck to play and I do think it is viable enough as well.

Let's talk about the roles of every single card:

CORE CARDS

Book of Specters: Your only spell in the deck (and only real draw engine), but it's so good. Incredible synergy with Mountain Giant (with Book of Specters, you can drop this on turn 3 if you have the coin), Twilight Drake, Meteorologist, Astromancer and Archmage Arugal.

Mountain Giant: Thanks to your handsize, this minion is always great in this deck. It's not easy to deal with. (Don't mulligan it unless you're 100% sure that your opponents plays an aggro deck.)

Twilight Drake: Hand synergy and dragon synergy in one. Need I say more?

Meteorologist: This card surprisingly does well in the deck. Incredibly strong with Brann Bronzebeard in the late game, you can even use it for lethal.

Astromancer: Power play on turn 7. Most of the time you get an 8 or a 9 mana minion.

Brann Bronzebeard: Irreplaceable card. More than half of the cards in the deck have synergies with Brann.

SUPPORT CARDS

Dragons and dragon-related cards support the deck very well.

Twilight Guardian: Very good midsize taunt, nice stat distribution.

Blackwing Corruptor: Can remove minions or push for lethal.

Dragonmaw Scorcher: Tech dragon against Paladin.

Crowd Roaster: Can remove big threats.

Sindragosa: Useful late game dragon that have some synergies in the deck (Frost Lich Jaina, Brann Bronzebeard etc). Also; fits the handsize gameplan. 

Firetree Witchdoctor & Netherspite Historian: Flexible support minions. Even if you play them, your handsize stays the same.

Scaleworm: Another flexible dragon-related card that can remove midsize minions.

Archmage Arugal: A decent value generator in the late game. Best to use with Book of Specters. Also, you can tempo-play on turn 2 against aggresive decks as well.

Frost Lich Jaina: Slow decks need super-strong late game cards and this is a no-brainer.

Tar Creeper: Good early game taunt that can slow down aggro decks and/or protect your other minions.

Emperor Thaurissan: Naturally a good fit for this deck.

SUBS/TECH CHOICE
Azure Drake
Primordial Drake
Kalecgos
Voodoo Doll

HAVE FUN Y'ALL!