Hwang is the best, stick with it through the torturous straight draw section in the beginning.
Rueben is good, read it next. Also he's so English it's awesome.
then Slotboom. He starts off with some short stacking stuff which is eh but has some good stuff from there.
You can buy the Ciaffone book but imo Hwang says the same stuff but better.
I got the Cloutier book against my better judgment and it's just as dumb as his NLHE book. It helps to understand the nits who only bet the nuts but probably you are doing OK vs them.
The section in
SS2 by berman is OK but if you don't already have it or have interest in the other games in there it isn't worth buying just for PLO, you can read it in like 45 minutes.