Let's be realistic here about why more people are buying Spellbooks:

Newly released Splinterlands features, such as land, Guilds, and the SPS governance token, may also entice new players to stick around, and increase Spellbook purchase rates.

That's about 99% of the story - no Spellbook, no airdrop.

I mean there's an almost perfect correlation between the day of the airdrop we're on and the rise of Spellbook purchases!