If the blog author holds the copyright on the ebook, they can distribute the ebook anyway they choose, either via download from Github or elsewhere, or sell it in an online store. They hold the copyright, they alone can decide how to distribute it. If they also sell the ebook in the Microsoft press Store, they may have an agreement with the store to also allow their own free downloads; it's hard to say without knowing the agreement.
If there are ebooks on Githb available for viewing or download, the owner of the Github page should hold copyright of the ebook, both 1) for their own protection against copyright infringement, and 2) to not break GitHub's TOS against copright infringement: https://help.github.com/articles/github-terms-of-service/#f-copyright-infringement-and-dmca-policy
REPRODUCTION OF THE DOCUMENTS IS AUTHORIZED ONLY FOR INFORMATIVE PURPOSES AND STRICTLY FOR PERSONAL, PRIVATE USE. IF YOU ARE AN AUTHOR OF SOME OF THE BOOKS AND YOU DON'T AGREE WITH PUBLIC ACCESS OF YOUR BOOKS, PLEASE CONTACT ME hi[email protected] AND I WILL REMOVE THE BOOKS FROM THE REPOSITORY.
Now Can I download for private use?