You're welcome to download the latest editions of any of PowerShell.org's free eBooks. Note that these are provided in DOC format, and you are welcome to convert them to whatever format you like (the free application Calibre is excellent for converting to PDF, EPUB, and other formats).
Books with "INPROGRESS_" in the folder name are totally incomplete. You shouldn't download those unless you're just interested in seeing what a half-finished, technically inaccurate book looks like as it's being written. Under no circumstances should you offer feedback on in-progress titles; it'll honestly just slow down the authors.
Otherwise, you're welcome to offer feedback, via the PowerShell.org forums, on the LATEST EDITION of any NOT IN-PROGRESS title. Please don't offer feedback on older editions - make sure you've downloaded the latest and greatest. You're also welcome to author additional content for any title. Each book has a curator, and they're responsible for incorporating new content. That's especially true for the "Gotchas" and "Practices" titles, which are really meant to be inclusive of communinty-contributed content.
Please note that this repo is not intended for public collaboration - only members of our author/editor/curator team have write permissions, and they use this repo to collaborate. For you, this is just a convenient place from which to download our latest efforts.
These works are licensed under a Creative Commons license (as detailed within each book's title page).
If you'd like to become a contributor, please contact Don Jones (@concentrateddon on Twitter).