I think it depends on what kind of documentation, if you want functional technical documentation, you can buy the odoo books, within the books there is also development documentation.
https://www.packtpub.com/catalogsearch/result/?q=odoo
Odoo as such also has official documentation, which to my liking is not so explanatory, but it serves as a basis.
https://www.odoo.com/documentation/14.0/
Unofficial documentation that is very good.
https://www.cybrosys.com/blog/
https://odoo-development.readthedocs.io/en/latest/index.html
https://odooforbeginnersblog.wordpress.com
With time you may find your own sources, I leave this as a base.
Greetings