About the documentation, on the Lazarus site we have Don Siders, who dedicates a lot of time towards the documentation.
For all I can say, he will surely welcome any serious help offered. If you have concrete ideas, and if you are willing to spend your own time, writing help texts, then you should contact him.
- Discuss with him which style is acceptable.
- Our docs are FpDoc based. But FpDoc has the abilities to add "topics" (Articles on any subject, linked from other help items).
If you want to contribute to the wiki => you are welcome.
As for help in "book form": Here too, anyone is welcome to write such a book.
Despite the point by PascalDragon, that the existing teams have little time already, the existing teams are software developers. So the skills in the teams are not the skills usually needed for writing "professional help/documentation books" (though of course I do not know to which extend individual team members may possess both sets of skills / Actually, some have done, see the bookstore).
There are also some free PDF books available.
But in the end, if more documentation (outside the FpDoc driven docs) is wanted, then it will have to come from contributors.