For a good reason, the Slotboom and Ciaffone books are the standard response to this question. If you have Super System 2 laying around, there is a chapter each for PLO and OMAHA-8. If you want more on OMAHA-8, you need Zee's Hi Low Split Poker book (which is better for STUD-8, but okay for OMAHA-
. Another consideration for PLO is How Good is Your PLO
, although I have heard that it is not good for beginning players.
I would also suggest looking into playing some 7-card stud if you are bored with hold'em. At the lower limits, online stud players are quite awful, so a beginning tight-aggressive approach, with some modifications if the game is loose (which is typical), is at worst
break-even. The only required reading is the stud section from Super System 1
and then Seven Card Stud for Advanced Players
. There is not much Stud-Hi reading to do besides those two books.
Want to play Stud-8? The Stud-8 section in SS2 is pretty good for as short as it is, but the best book to read is again Zee's Hi Low Split Poker.
Want to play Razz? Read Sklansky on Poker