Here is a link to some of my books:

www.researchgate.net/publication/234128426_Adivina…

¿Adivina para qué son las matemáticas?
Shared under CC BY-NC license (Attribution, non-commercial)

www.researchgate.net/publication/377411029_Guess_w…

(It's translation to English, again shared under

CC BY-NC license (Attribution, non-commercial)