I'm always a fan of cards as record-keeping in games as they make the game flow much smoother and tend to support players to be more prepared (and the DM to construct material on the fly). These cards are excellent, providing all of the spell information you need for your 5e game. I'll not only encourage my players to purchase a set to have at the table (and construct custom decks to represent their spellbooks or equivalent) but also as a tactile treasure when they lay hands on a captured spellbook. There is an extra sense of achievement for the player when you hand them a small stack of cards to rummage through after defeating an enemy mage and these are perfect for the job.
An accompanying product that I'd buy in an instant would be collections of monsters on these sorts of cards (like the old 'Monster Sets' from first edition), perhaps as themed decks (ie 'Dungeon Monsters', 'Aquatic Monsters', High-Level Threat Monsters'). I could imagine combining some monster cards with a few spell cards to make book-keeping and encounter construction behind the screen much easier.
Keep up the good work, Myth Merchant Press.
[4 of 5 Stars!]