Yes, you can earn glint just by winning games, without any investment. It will be slow but it's nice to try the game out. You can even join the Splinterlands discord and let people know you just joined, you'll likely get some cards delegated to you for free. That means you can use those cards until the owner decides to take them back.

Spellbook is a $10 purchase that allows you to earn crypto in the game. It also gives you 3000 credits (worth $3) and you even get a temporary SPS delegation that lets you start earning some SPS on top of the Glint.