Siam's participation in Imjin war

This is very interesting indeed. I had absolutely no idea. What is the name of the book, so that I might keep an eye out for it (though there seem to be very few books on the Imjin War here)?

I imagine that the logistical difficulties involved with sending more than a token force from Ayutthaya to Korea would have been immense, however.

I read something about that recently as well.

At the time, Ayutthaya has recently been liberated from Burmese domination. Naresuan would want to reestablish Ayutthaya's power in the region. Ayutthaya still faces a still-strong Burma, as well as threats from Lanxang and Vietnam. Helping the Chinese (to whom Siam and other SE Asian kingdoms sends tribute) against the Japanese would be something that would strengten Siamese pristige and helps to build stronger relations with China.

At the time of the Imjin War, trade with Japan is limited and mostly conducted through the Ryukyu Kingdom (a tributary of both China and Japan), which is unlike the situation in the early 17th century when trading links between Ayutthaya and Japan were much stronger. Naresuan thus had little to lose if he attacks the Japanese. In the end though, since Siam is considerably far from Korea, the theatre of war, the Emperor decided the Siamese won't be much help in the war and declines the offer.