100 decks surveyed, top 10 answers on the board...
While 100 decks were surveyed, only 16 decks were monosophere, and two of those were before the AtS cycle, which has almost all the monosphere cards. Fifteen of those were three-hero monosophere, and one was a lone-Aragorn secrecy deck.
Much like multiplayer, we're not looking specifically for cards that are better in monosphere, but rather cards that essentially require monosophere by either printed symbols or by using three hero's resources.
3x Emery (Lo3) [3/3]
3x Knight of Minas Tirith (Ta3) [3/5]
3x White Tower Watchman (N3) [3/14]
Emery is essentially free in a monosphere deck, so the popularity is easy to explain.
Knight of Minas Tirith is an exception to the rules above, since he has a "when each has a tactics icon" ability to add to his Gondor 0/3/1/3 stats. In monosphere tactics, he can attack an enemy in staging when played.
White Tower Watchman also has a monosophere ability to go with his Gondor 1/0/2/3 stats. He can absorb undefended attacks.
4x Strength of Arms (Le2) [4/4]
1x Trained for War (Ta2) [1/5]
0x Reinforcements (Le3) [0/1]
0x Shadows Give Way (Sp2) [0/3]
0x Advance Warning (Lo3) [0/3]
0x Against the Shadow (Sp2) [0/3]
0x Thicket of Spears (Ta3) [0/5]
The events restricted to monosphere should be so powerful enough to make a home in monosophere decks, but that's not largely the case. We're talking about small samples, but five of the cards whiffed completely. Trained for War did get used by one of five possible decks, able to turn a questing quest into a battle quest, for a turn. If only it were an attachment, it might actually make solo Tactics viable.
The one big success is Strength of Arms, which was found in all possible monosphere Leadership decks.
100% Strength of arms (4 event) [4/4]
00% Reinforcements (3 event) [0/1]
00% Advance Warning (2 event) [0/3]
100% Emery (3 [free] ally) [3/3]
00% Shadows Give Way (3 event) [0/2]
00% Against the Shadow (2 event) [0/3]
60% Knight of Minas Tirith (3 ally) [3/5]
20% Trained for War (2 event) [1/5]
00% Thicket of Spears (3 event) [0/5]
21% White Tower Watchman (3 ally) [3/14]
You can also check the Record attachments - their cost of 2+ in non-monosphere decks is usually not justifiable. I'd consider that most decks using them would be monosphere.
Also, Mithrandir's Advice is bananas powerful in mono-Lore.
I'm not convinced the books are necessarily a bad value at 2 if you have two of the hero. I actually run it in my spirits/tactics deck because the tactics resources sometimes pile up, and Tactics has some awesome cheap events. They're essentially Dwarven Tomb in attachment form for the same cost as Dwarven Tomb monosphere, and one more with two heroes.
Here's the books popularity by sphere:
4x Book of Eldacar (5 mono-tactics eligible)
2x Tome of Atanator (3 mono-leadership eligible)
1x Map of Earnil (2 mono-spirit eligible)
0x Scroll of Isildur (3 mono-lore eligible)
It makes sense that Scroll of Isildur is the neglected child here. Tactics and Spirit both have great cheap events, and Leadership has Sneak Attack, but Lore's most popular event by far is Daeron's Runes, and that's probably not a compelling enough card to rate breaking out the Scroll of Isuldur.
The various books may get a boost even outside monosphere from the new Noldor-friendly events in Gray Havens, though. They get better with copies in the discard, and the book can recycle them if you use them all, to good effect.
Mirthrandir's advice is a great example of a card that shines in mono lore, but at 2 card for 1 cost it's playable with two lore heroes.
5x Mirthrandir's Advice (2 mono-lore eligible)