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