now i read the book seven card stud for advanced players and sklansky recommends in this book . after a raise with JhJs9c and a reraise from a king to fold his jacks. isnt this to nitty in this spot?

Depends on the opponent. If he's not raising as a bluff, what is his range? What's your equity against that range? (use propokertools.com)

Depends on the door-cards (dead-cards) and how many other players might join the pot, their position and ofc all that Rusty already said.
The Jacks are worth more, because they are hidden, imo.

Here is the link to http://propokertools.com/simulations

It depends on opponent tendencies and live/dead cards.

Recently I folded Jacks to heavy 3rd street action.

I had J-8-J

A player with x-x-10 completes, I call with J-8-J. A player behind me raises with x-x-6.
When the action gets back to x-x-10, he reraises. My intuition tells me that the player with x-x-6 is raising behind 2 players with a 6 showing and what I know of this opponent is that he usually makes this type of play with big wired pairs.

I folded and probably good that I did. When it went to showdown, it turns out that x-x-10 had K-K-10 and made Kings up. X-X-6 turned out to be A-A-6 and he won the pot at showdown with 3 of a kind Aces.

So, I feel by knowing tendencies of your opponents, as well as considering live and dead cards, you can save a few bets here and there by folding Jacks and other pairs in some cases.

Why call xxT with a pair of jacks? Obvious spot to raise.

Chip Reese said it more succinctly, don't get in with a second-best hand. You can make occasional exceptions when you have more ways to improve than the overpair does. If you open J9J unsuited, you see a Queen and Ten out, and a King reraises, you fold because your only secondary equity (straight draw) is badly compromised.

If you have (JhJc)9h and a King reraises you when there's a King dead, your suit is live and no Eights, Tens or Queens are dead, then you can call the reraise and decide on Fourth. If you pick up additional equity (heart, Ace or straight card) then you continue, if not fold. If you make concealed trips raise and prepare to win a big pot.

If you have (JA)J then you can continue and usually be prepared to show down any improvement unless the Ace falls dead or villain's board becomes insurmountable.

I called because this opponent is really loose and gets easily trapped by slow play. He is the type of player who overplays medium hands, essentially building the pot and trapped on later streets.

just calling might encourage other calls though and a pair of jacks plays much better in a headsup pot than in a multiway

I know this, but there was only one player behind me with xx6 and the bring in, and the player with xx6 is a fairly tight player, so chances of him calling here are small. If I was in an early position on 3rd with many players yet to act, then I would raise to discourage too many callers.

In the game I play in I would need a pretty awesome read the player had kings to fold J-J-9 heads up on 3rd street after I opened with a 9 showing, all else being equal.

I have been in this spot against players I thought were solid/predictable and seen a whole lot of stuff, including smaller pairs with K kicker, 3 flushes, AKQ type hands, etc.

Unless you are super predictable yourself, he could also be re-raising with hands that are ahead of your range (ie buried 10s or 9s), where he thinks he actually has the best hand.

In a vacuum I would definitely call 3rd, I would call 4th unless he caught real good (pairing the king, catching a suited Q, etc.) and then re-evaluate on 5th depending on what we caught and reads.