Librams are paladin-specific relics which grants bonus to a certain spell or ability of the paladin. Librams are only obtainable at level 52 through instances runs, more will be available once you reach Outland content. Here I will list only librams there are useful and easily accessible through 5-man instance runs, since 40-man raids such as AQ40 are pretty much dead now.

Libram of Hope

Binds when picked up
Requires level 57
Equip: Reduces the base mana cost of your Seal spells by 20.
Drop by Isalien of Dire Maul

Seals are the primary offensive spell of the paladin, so this helps with mana conservation especially if you are a Retribution paladin who constantly uses Seal of Command.

Libram of Divinity

Binds when picked up
Requires level 60
Equip: Increases healing done by Flash of Light by up to 53.
Drop by various bosses in Scholomance

A good libram for healing and better mana efficiency. The sad thing is that it only buffs Flash of Light, but useful nonetheless.

Libram of Saints Departed

Binds when picked up
Requires level 61
Equip: Causes your Judgements to heal you for 41 to 49.
Drop by The Maker in The Blood Furnace

Good for the Retribution and Protection paladins who judges a lot although the amount healed is small. This libram is more for leveling purposes, goes well with Protection AoE leveling.

Libram of Zeal

Binds when picked up
Requires level 62
Equip: Increases the melee attack power bonus of your Seal of the Crusader by 68 and the Holy damage increase of your Judgement of the Crusader by 47.
Zangarmarsh PvP Reward

Useful for the Retribution paladin in leveling or PvE, but not for PvP.

Blessed Book of Nagrand

Binds when picked up
Equip: Increases healing done by Flash of Light by up to 79.
Quest Reward from: The Ultimate Bloodsport

An upgrade from the Libram of Divinity, you should at least have this if you intend to start healing in Karazhan.

Libram of Eternal Rest

Binds when picked up
Requires level 68
Equip: Increases the damage of your Consecration spell by up to 47.
Drop by Darkweaver Syth of Sethekk Halls

Great for Protection paladins for tanking as additional holy damage allows them to hold aggro. This is also good for paladins who have high spell power like Shockadins.

Libram of the Lightbringer

Binds when picked up
Requires level 70
Equip: Equip: Increases healing done by Holy Light by up to 87.
Drop by Commander Sarannis of The Botanica

Another must-have libram for healers starting to raid. Holy Light is better for healing tanks against bosses who deal high burst damage.

Libram of Righteous Power

Binds when picked up
Equip: Increases the damage dealt by Crusader Strike by 18.
Quest Reward from News of Victory (Alliance, Horde)

Specifically for Retribution paladins, sadly the additional 18 damage is considered insignificant. But if you are lacking a Retribution libram you can get this.

Libram of Avengement

Binds when picked up
Requires Level 70
Equip: Causes your Judgements to increase your Critical Strike rating by 53 for 5 sec.
Drop by The Maker in The Blood Furnace (Heroic)

A very powerful libram for Retribution paladins, this gives additional 2.4% critical strike chance when the paladin casts Judgement. Retribution paladins reply on critical strikes to improve their damage so this is a boost. Also this should be a compulsory libram for Retribution paladins starting on Karazhan. Note that this drops from Heroic Blood Furnace, where revered reputation with Thrallmar/Honor Hold is needed before you are able to enter.

Libram of Souls Redeemed

Binds when picked up
Requires Level 70
Equip: Increases the benefit your Flash of Light and Holy Light spells receive from Blessing of Light by 105.
Drop by Opera Event Boss in Karazhan

The only libram to drop in Karazhan and specifically for healers. This further boosts the healing bonus given by Blessing of Light and is excellent for healing tanks. However in most raids Blessing of Kings is favored over Blessing of Light. So I suggest it is best to use it only when you have at least 2 paladins in raid, 1 can cast Blessing of Kings while the other equipped with this libram can cast Blessing of Light. This combination of blessings helps improve the raid alot.

Libram of Repentance

Binds when picked up
Requires level 70
Equip: Increases your block rating by 40 while Holy Shield is active.
Vendor Reward from G’eras of Shattrath City

With 15 Badges of Justice you are able to purchase this in Shattrah City. A must-have for serious tanking paladins, this adds approximately 5% block chance when Holy Shield is active.

Libram of Justice

Binds when picked up
Requires level 70
Equip: Causes your Flash of Light to increase the target’s Resilience rating by 26 for 6 sec.
Vendor Reward from Vixton Pinchwhistle (Arena Vendor) in Netherstorm

For the paladin healer in Arenas, this grants a boost to Resilience to fellow teammates, preventing them from suffering critical strikes and reducing damage taken from DoTs.