To begin with you have the UI skinning tutorial wich explains the basic concepts of skinning LotRO and some of the basic ID's:
http://www.lotrointerface.com/index.php?p=ui_skinning_1
Then you have the extended (complete?) set of art assets ID's in this post:
http://www.lotrointerface.com/forums...hread.php?t=70
To learn more about the positioning and resizing made available with the Book 10 patch you can read Froty's post here:
http://www.lotrointerface.com/forums...read.php?t=250
You can also download the lastet (preview) art-pack wich includes all art assets to create the default skin (wich is a great starter and contains all ID's):
http://www.lotrointerface.com/downlo...tPack.zip.html