if you are locked in at steam you can buy the supporter edition bundle, without paying the parts you already have. In your case you have only to pay the lore book, fangshire helm and the soundtrack and not the game. The lore book is not available for single purchase, to my best knowledge.
If you have the money by all means buy it. The soundtrack alone is worth it, because it is freaking awesome! I don’t know how often I heard ‘Warpath Ceremony’ or ‘Thug Life’ without actually playing the game. Have fun!
Grüße von der Ostseeküste,