Highly recommended references on Modular Forms

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I was reading this following attached below part of the asymptotic winding of the geodesic flow on modular surfaces and continuous fractions article, Y. Guivarc'h and Y. Le Jan. enter image description here enter image description here To be honest, I know nothing about Modular Forms. Therefore, I would be very grateful if one could suggest highly recommended references on Modular Forms. (I am not required to master this area of Mathematics, but to understand, at least, this part of the article).