Enclosed a link collection of my mostly used reference books, sorted by topic:

Powershell Tutorials:

https://www.guru99.com/powershell-tutorial.html
https://www.petri.com/category/powershell
https://ss64.com/ps/
https://powershell.org/category/tutorials/

https://learn-powershell.net

https://4sysops.com/tag/powershell/

https://powershellexplained.com/blog/
https://www.windowspro.de/search/node/powershell
(German Powershell)

Function / Script librarys:

http://dennisspan.com/powershell-function-library/
https://www.powershellgallery.com/

Citrix:

https://www.carlstalhood.com/
https://dreadysblog.wordpress.com
https://carlwebster.com/

Linux:

https://wiki.debian.org/