so i was reading the book and it said ulfric was a thalmor asset. does this mean he is in league with the dominion?
also the last line of the book says "A Stormcloak victory is also to be avoided, however, so even indirect aid to the Stormcloaks must be carefully managed." In my opinion this goes against the theories of the thalmor inciting the rebelion. can someone explain this?

Asset =/= Agent.....

As long as the civil war continues the Thalmor benefit from it since Men are dying.

Just because the Thalmor consider the Stormcloaks an asset in their political machinations does not mean that they are working with them directly. The Stormcloaks hate the Thalmor, as does the Empire.

He’s indirectly aiding the Thalmor by turning the majority of the Empire’s attention towards Skyrim and away from the Dominion. Like how lil_rhy stated, as long as the civil war continues, men from both the Empire and Skyrim will die, the more the Altmeri will gain.

It would be most accurate to classify Ulfric as an Unwitting Pawn. Not a Manchurian Agent variety either. Quite simply, his anger is manipulable since the Thalmor had plenty of time to study his psyche while he was a P.O.W.

Being an asset does not mean he works for the Dominion, it just means that the Thalmor profit from the civil war.

This.
Also it demonstrates the short sight of Ulfric regarding strategy.
Starting a civil war in that case, just plays in benefit of the thalmor, weakening both, the cyrodiil and skyrim forces.

The only way the civil war helps the Thalmor is if it continues. If Ulfric wins, then theres a bit of a kink in their plans having an entire nation of elf hating Talos worshipers openly challneging them every step of the way.

But, taking in count how the game goes, the war just change tides with the player intervention.
So in the original case, it was tied, and both forces were just killing each other slowly.