[Wien] confusion about the caculation U in LDA+U method

[白宇]

Dear WIEN users:
I read the articles which descible the theory caculation about the U by Mr.Anisimov and WIEN caculation of U of Mr.Novak, but still I don't quite get it.Can you suggest me some papers and books that
can help me get more insight in this theory?

In addition, whether the U is depend on the overall molecule electron system,or just the transition mental atom. Which I mean I found the value of U of the transation metal in other system, can I use it in my own system?

Farther more, is it a reasonable way I add this U to the GGA exchange potential? Or does the so called GGA+U using a different U?

Thank you very much.