Preheat oven to 425°.
Combine spinach and ricotta cheese in a small bowl. In a separate bowl, combine beans.
Cut 3 corn tortillas into pieces, and arrange in a solid layer to cover bottom of an 8-inch square baking dish coated with cooking spray. Top with half each of beans, salsa, spinach mixture, and cheese. Repeat layers with remaining ingredients.
Cover and bake at 425° for 25 minutes; uncover and bake an additional 15 minutes. Let stand 10 minutes before serving.