Lord Vishnu and Lord Shiva are probably most famous in Hindu stories, but I'd like to also know if there are any books like the upanishads that describe the leelas of mother Saraswati?
Yes, there is Sarasvati-rahasya Upanishad which means “the Secret Knowledge of the Wisdom Goddess” This is the medieval era text and one of the minor Upanishads of Hinduism. It is classified under the eight Shakta Upanishads and Krishna Yajurveda. It's text worships Sarasvati form of Shakti which is Hindu goddess of arts, music and learning.
These manuscripts are also entitled as Sarasvati Upanisad, Saraswati Rahasyopnisad, Sarasvatyupanishad and Sarasvatirahasyopanisad.