4 out of 5 rating for Spellbook Cards
Spellbook Cards come in packs based on class (except for the Arcane pack, which contains all wizard, sorcerer, and warlock spells). The cards are almost identical in size to Magic: the Gathering cards, so there are myriad options for sleeving. Also, since you are unlikely to shuffle Spellbook Cards, their square corners will likely only be an issue if you abuse them while unsleeved. Lastly, while some have noted there is no specific notification for spells with a Concentration duration, the reality is that every card with an 'up to' duration is a Concentration spell except for two cantrips -- Prestidigitation and Thaumaturgy. (This terminology exactly matches the Player's Handbook, so it's not GF9's fault, really.) Only significant downside is that many classes and some feats grant spells outside of a specific class to other classes, so you may end up buying an entire set of cards simply for access to one class-specific spell.