Backgammon- have read 501 and for serious players, what next?

[lwspoker69]

So I've read 501 essential backgammon problems and backgammon for serious players. I'm eager to tackle some more backgammon literature. However, I don't see anyone here recommending anything past the 501 essential bg probs level. The 2-book advanced backgammon set by Robertie any good? I really am clueless from here on out, so any help would be great.