Haven't found any books for the current version 5.7. Is there any I missed? I understad that this version is largely rewritten so maybe books for older version will only be misguiding.
With kind regards, Johan
I am not aware of any books covering 5.7.
There seems to be enough changes from previous versions that reading older books might lead to confusion.
If you are an experienced developer, I would read through the documentation, look through the core to see how things are made and work, and start browsing the API.