Since Shad0w has helpfully posted the top books from GenCon 2014, I thought it would be interesting to look at the spells the top 4 books had in common. (I remember way back there was a topic of common spells that all books should have, and I thought this was an interesting way of examining the question.)
The following are spells found in all 4 books:
1x Geyser
1x big attack (Hurl Boulder/Fireball)
1x spawnpoint (Lair/ Battle Forge/ Wizard's Tower/ Gate to Voltari)
1x torso armor (Bearskin/ Dragonscale Hauberk/ Stormdrake Hide)
1x Leather Boots
2x Dispel
1x Seeking Dispel
2x Dissolve (& +1 Dissolve or Acid Ball)
2x Teleport
2x healing sources
I think it's safe to say every book should 100% have those. (Though possibly Teleports could be Force Pushes?) It seemed to me a few cards that are very good got skipped because one person did something unusual. So, I tallied all copies between the spellbooks to find a ratio per book:
1.75 Acid Ball
1.25 Geyser
3.50 level 1 attack spell (Arc Lightning / Jet Stream / Hurl Rock / Flameblast / Surging Wave)
2.75 level 2 attack spell (Fireball / Hurl Boulder / Lightning Bolt / Thunder Bolt)
1.25 Mana Crystal
0.75 Mordiok's Obelisk
2.50 spawnpoint
1.25 wall
2.25 level 1 creature
1.00 level 2 creature
1.25 level 3 creature
0.75 Harmonize
0.75 Hawkeye
2.00 Nullify
1.00 Rhino Hide
2.00 torso armor
1.00 Elemental Cloak
1.25 Elemental Wand
1.00 Leather Boots
0.75 Leather Gloves
1.25 Mage Wand
1.25 Suppression Cloak
3.75 Dispel
3.75 Dissolve
1.25 Force Push
0.75 Minor Heal
0.75 Purge Magic
0.75 Purify
2.00 Seeking Dispel
2.25 Teleport
1.75 passive healing (Renewing Spring / Regrowth / Regrowth Belt / Sunfire Amulet / Wand of Healing)
I think some of the counts are skewed heavily by having 3 Wizards in the top 4 (attack spells, conjurations, and enchantments), but that in other areas the card counts seem like a very good general rule of thumb (equipment, incantations).
I hope this was interesting for everyone!
Edit:
Some people asked for a report on the Top 8, so I'm adding it below.
Spells appearing in 100% of top 8 spellbooks:
1x big attack (Hurl Boulder/Fireball/Force Hammer)
1x spawnpoint
1x torso armor
2x Dispel
2x Dissolve
1x Teleport
2x healing sources
Ratios of popular spells to Top 8 books:
1.25 Acid Ball
2.37 level 1 attack (not including acid ball)
2.00 level 2 attack
1.25 Battle Forge
2.37 mana manipulation (5 Mana Crystal, 1 Mana Siphon, 5 Mordiok's Obelisk, 3 Harmonize, 6 rings, 1 Psi-Orb)
1.00 spawnpoint (1 Gate to Voltari , 1 Lair, 1 Libro Morous, 5 Wizard's Tower)
1.25 wall
1.12 level 1 creature
1.62 level 2 creature
1.37 level 3 creature
1.87 Nullify
0.87 Leather Boots
1.75 torso armor
2.25 sources of healing (Drain Life, Healing Spring, Wand of Healing, Regrowth, Regrowth Belt, Minor Heal, Sunfire Amulet)
3.25 Dispel
3.37 Dissolve + Explode (25 Dissolve, 2 Explode)
1.50 Force Push
1.62 Seeking Dispel
2.12 Teleport
Hmm, pretty interesting! Good idea Wildhorn.