I don't know about omaha books, but there is newly created omaha course on upswing poker:
https://www.upswingpoker.com/plo-pot-limit-omaha-education/
This is very expensive (999 USD atm), but if the quality is as good as on holdem poker, it is probably worth it. I guess that recreational players won't be able to afford it, but if you are living from holdem and want to transition, it might be worth a shot. I was very excited about this course and then I saw the price and you know, sad panda, but what can you do
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