SEO and WordPress-theme for a digital handbook

Hi all!

I need some help from you skilled guys concerning a new project of mine.

I am in the process of building a digital handbook accessibly on a url in a subject that concerns many people. The subject is quite complex and is very broad.

The most important function of the handbook is that people that are googling there problems and questions concering this subject will actually find the handbook on google, hence SEO has top priority. After SEO the next top priority is usability for the person reading the handbook.

I need help with how to construct/build this handbook from a WordPress and SEO-perspective.
Those anyone have any suggestions for a suitable WordPress-theme? Or a plugin. Or another website that I can look for inspiration?

Would be most happy for any kind of feedback concerning the above!

Thanks in advance for your help

Kind regards,