DISCLAIMER: Please note, this is based purely off of personal opinion and in-game experience. I take half of the credit and give the other half to Blizzard.
FIRST TIMER'S GUIDE FOR SCARLET LIBRARY
So, you want to do the Scarlet Monastery's Library, run by that total sonuvabitch Doan. Here's something to start you off.
IDEAL PARTIES FOR ALLIANCE
32-40 Warrior (MT)
32-40 Paladin (Subtank and Secondary Healer)
30-40 Mage (Nuking and DPS)
30-40 Priest/Druid - Priest recommended (Primary Healing)
30-40 Wild card (Hunter or Warlock)
34-40 Warrior (MT)
30-40 Warlock
30-40 Mage
33-40 Druid (Primary Healer)
34-40 Rogue
IDEAL PARTIES FOR HORDE
32-40 Warrior (MT)
33-40 Shaman (Secondary Healer and Sub tank)
30-40 Mage/Warlock
32-40 Rogue
32-40 Wildcard (Hunter or another Warlock)
32-40 Mage
32-40 Warlock
34-40 Shaman
34-40 Shaman
32-40 Priest/Druid/Warrior
These are the parties that have been most successful for me, and as said in the disclaimer, the parties vary greatly and this is based off of my experience.
STARTING THE INSTANCE
Now, usually the only reason you're here is to get some fat loot (e.g. Mantle of Doan, Illusionary Rod) or because you need the key, or you can't do Armory/Cathedrel yet. So, when you start off, there is a hallway chock full of enemies. Some tips for this area.
ALLIANCE: Now, the pally wants to get Judgement on Justice when an enemy is maybe half health. Hammer of Justice works well, too. Since this is a pretty full hallway, you need to keep them snared to stop them from getting more buddies. Frost Nova, and possibly a Challenging Shout work well. Or, if you're feeling stupid and suicidal, or you feel like you can take them all, get your mage to AoE, Frost Nova, rinse, repeat. Tanks, you'll need to keep the aggro pretty well so you don't get a TPW. And that makes it even worse if you are using the second party formation, since the Druid has the disadvantage of one resurrect every half hour. Warlocks, you do NOT want to Fear. Surprisingly, in this little hallway, Fear is your greatest enemy (no pun intended), since it can send your enemy sprawling across his or her buds, drawing even more aggro.
HORDE: Earthbind and other snares work wonders here, your shaman wants Frost Shock ready so the wimpy crusaders don't run off and get more buds. If you're using the second party formation, the Shamans are charged with tanking, which can be rather tricky since healing is a hassle during combat. However, you gain the advantage of two nukers, which can be a great boon. Unlike the Alliance, AoE without the first formation can be a real pain, since your main tanks wear leather. However, if they don't guzzle their mana like a thirsty child with a full soda can, then you should be able to get through without any casualties. But always carry some health and mana potions.
THE COURTYARD
ALLIANCE: Now, like the previous hallway, this courtyard can be a little tricky. You don't want to aggro anybody in the courtyard just yet; this is when Judgement of Justice, Hammer of Justice, Hamstring, etc. comes in handy. Its not a bad thing to aggro the Houndtrainer people, although it can be more difficult with galants and hounds with Houndtrainers pegging you with wooden shafts tipped with sharp pointy things. After clearing the ring around the courtyard, you can smack the people in the courtyard. Yes, its very vague, but due to the fact that everybody is different, its as general as I can be.
HORDE: Pretty much like alliance, except you might have a better chance tackling the courtyard first. Since the Courtyard is populated with victims, AoE works well here, and the Shaman's totems get enough targets. After clearing out the courtyard, give a whacking (no, not the kind you think) to the Crusaders around it.
BOSS: HOUNDMASTER LOKSEY
So, now you need to kill that damn Houndmaster and his yipping, red, deadly excrutiatingly large Pomeranians.
ALLIANCE: For this part, your main tank will actually want to engage the hounds, while your sub tank and the rest of your party hammer Loksey with spells, Judgements, prayers of pain, etc. Paladins, since Stuns are always useful, especially during boss battles, you might want to save that Hammer of Justice for Loksey. Priest, don't neglect the warrior. If he dies, and the hounds are set loose on you, its all over. Then again, if a tank dies in any instance situation, usually the party is screwed anyway. But that's not the point. With a Rogue, you'll want to sap one of Loksey himself, then kill his Pomeranians, then smack Loksey. Usually you'd use strategies like sheeping one of his dogs, but you want to kill them and Loksey ASAP.
HORDE: Formation one is pretty much like Alliance, but Formation two can be somewhat tricky. One of your shamans wants to engage the dogs, and you'll want to kill his dogs FIRST. Loksey himself is actually no real threat, what turned out to me was that his dogs were the bosses, not Loksey. Lay down Stoneskin for some extra armor, maybe with Searing or Magma totem. Shammys, be sure to constantly enhance your weapons. After killing Loksey's dogs, go after the Houndmaster himself, and ninja the Dog Whistle.
MORE HALLWAYS AND THE GALLERY OF TREASURES
ALLIANCE: Now, how you deal with more hallways is up to you, whether you follow the step for the first hallway or you create your own way to wipe out the Crusaders. Not much to say. The Gallery of Treasures, however, might be a little tricky. Warriors and Paladins want to interrupt as much spells as possible with Shield Bash, Pummel (if you have it), and Hammer of Justice. Again, keep them snared, and if you want, get the warrior to aggro them all while your PH heals like mad.
HORDE: Now, this is very similar to the Alliance, with the exception of being able to use Hammer of Justice. Earth Shock is actually, more or less, useful in interrupting spells, because it doesn't bring a 1 minute cooldown on your shoulders. Searing is a good totem to bring a killing blow to the fleeing Crusaders. Again, don't use Fear in these kinds of contested areas unless you are fighting the last enemies in the room. Rogues can be useful with their snaring abilities such as Gouge, and starting off with a Sap gives you a good edge, or using a Polymorph.
BOSS: ARCANIST DOAN
Good job, you made it to Arcanist Doan! Now all you need to do is beat down that sonuvabitch, ninja the loot, and hearthstone out.
ALLIANCE: First, I always mock Doan by saying "Yes we will!" After he yells "You will not defile these mysteries!" Spell interrupting is imperative. Stuns also work so he doesn't use Arcane Explosion. If Doan just uses his staff on you, he's a cake walk. You'll want to interrupt Flame Strike, since it can do a good deal of damage. Pallies, turn on Seal of Justice to get a stun chance.
HORDE: Formation one = refer to alliance section. Formation two = slightly difficult. Earth Shock is a party's best friend here for the spell interuption, and Tauren want to use War Stomp to keep Doan in one spot. Lay down Stoneskin with a mixture of offensive and healing totems to help out. Press Need on the loot, put those members on your ignore list, leave the party and hearthstone out.
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