The Openstax organisation offer free books that are peer-reviewed by scientists and other academic individuals. There are currently a number of books available by them related to biology but they have now launched a new microbiology book well worth having a read of. It may help you with your studies. You can download the Microbiology book at the link below:

https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Microbiology-OP.pdf