Some more details on the written content improvement

[therahedwig]

So, doing the book research for the wiki, I ended up staring a lot at the contents via lua.

Here it is said everything above 0 is ignored.

https://github.com/DFHack/df-structures/blob/ff790ff50ddc26cc90fd184cbd7c10964fa9b7df/df.itemimprovements.xml#L186

It seems that... 0 indicates that this is the main topic. Some books, like encyclopedias have multiple main topics. However, only the manual seems to teach anything. I suspect the learning about sites/people/regions/figures/events from books (and slabs, now I think about it), is not implemented at the moment. 1-4 I have not seen. 5 is for chronicles, biographies, autobiographies and cultural histories, the events here get written down as 'the first chapter concerns event x', so I suspect these are about chapters within a work. 6 is for poems, seems to in particular be about the values these espouse??? I can't find the description text of poems which have these refs. 7 is referencing poetry, this happens with musical compositions. Can be multiple. 8 is for choreographies, and references music compositions. 9 is for when academic works espouse a value.

I am still trying to figure out how value in written content exactly functions. I doubt only essays and dialogs (which have values as their main topic) adjust personal values. It's a bit tricky: histfigs mostly really like writing manuals and poems, and only manuals are bound to be written onto something.