I need to constantly monitor the state of some selected nodes. When they'll change their values i'll have to get this changes.
Infinite run loop with values extraction from nodes is impossible, because this will drop performance heavily. Everything i've found is a getCookCount method of OP_Node. Each node will have it's own cook count, so i'll have to maintain a list of previous cook counts and nodes. That's a solution, but is there any better solution?

Are you doing this in Python/HOM or in C++/HDK ?

In Python/HOM, you can use hou.Node.addEventCallback(). In C++/HDK, you can use OP_Node::addOpInterest().