I have Super Sytstem and would like to buy another book. Which book by Harrington is the best for cash games?

harrington are not for cash games... they are written for tournaments. But if i had to pick up one i guess it would be the 3 because it explains a lot how to think a hand which it's a skill important in cash.

Yes, all three are geared towards tournaments, but I think the first one could help for cash games. A lot of it relates to the early stages of tournaments when the blinds are low and you can play your normal game.

I think Harrington is bringing out a book on cash games this year. That's what I heard anyway.

So, what is the best cash game book for Hold 'Em?

Well I just this minute ordered Low Limit hold'em by sklansky. Also heard theory of poker by sklansky is useful for cash games.

For No-Limit, probably No Limit Hold 'em: Theory and Pracitce by Sklansky/Miller. For Limit, Hold 'em Poker For Advanced Players by Sklansky/Malmuth.

A bit of warning, NL Hold'em Theory and Practice is a great book, but it's definitely for advanced players.

yeah, NL theory and practice is good but no it doesn't really help that much if you intend to 6max NL
There's no really good books for NL cash 6 max as they are for limit poker or tournaments.
I think the fastest way to improve your game is to get a coach if you can afford it, or subscribe to some pro videos like cardrunners