A good starting point (that has lots of references you can also study) is here: A Review of Digital Techniques for Modeling Vacuum-Tube Guitar Ampliﬁers
For some deeper reading (and many more resources to research) you can check out the PhD Thesis of David Yeh, one of the authors of the above: "DIGITAL IMPLEMENTATION OF MUSICAL DISTORTION CIRCUITS BY ANALYSIS AND SIMULATION"
There is a lot of fragmented information out there; if anyone has some more consolidated links please feel free to post them.
I will add knowing how a tube amp operates and how they are designed can give you a lot of insight on how to model them. There's a wealth of information here:
I can also highly recommend his books. Knowing how to read tube data sheets and load lines can help a lot. You can find most data sheets here:
Lastly, this forum is where guys that build and design amps hang out:
This is a thread which lists reading material for tube amp theory:
Most of my experience comes from building real amps. I've just purchased Will's book in hopes of creating some plugins built from real tube amp measurements. For example, depending on the rectifier type and filtering, a tube amp power supply will sag different amounts, which changes the transfer characteristics and operating point of each individual tube.
Hope this helps.
Here are a couple of newer sources - both very interesting; also drill down into their sources:
REAL-TIME GUITAR TUBE AMPLIFIER SIMULATION USING AN APPROXIMATION OF
REAL-TIME SIMULATION OF A GUITAR POWER AMPLIFIER
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