Not one of the most famous museums in the Madrid and definitely not a museum you’ll find in your Lonely Planet, but the great news is . . . this museum is always free!
Yes, every day of the week and it has a collection of illustrations, both in the exhibition and at the library shop, that will make avid collectors lick their lips in anticipation.
The complete name of the museum is the Museo ABC de Dibujo e Ilustración, directed by the Fundación Colección ABC (from the very famous ABC Magazie).
They have the biggest collection of drawings in Spain. More info here: Museo ABC.