Previously I was asked for some options for Guided Reading in an online format. Here are a few I found that include free books. These books would be easily shared with your students in Google Classroom. You could create a topic under the Classwork tab and share them as resources for your students to access anytime you need them to.
WillowBooks has quite a few books available for free especially in the lower levels.
Dr. Burkins has a collection for you and your students as well.
There is always the wonderful Epic Books. Which offers thousands of online books for your students to access for free from school.