The men were already boiling in from the field when he reached the yellow-domed room.
What does "boiling in from the field" mean in this sentence?
Laure's answer has covered most of the key points, but has missed an important figurative use of the word boiling, which is how (I think) it is being used here.
While boiling is most often used to refer to temperature (boiling hot), or figuratively associated with an agitated, "hot" emotional state (boiling with some emotion, e.g. rage), it can also be used figuratively to describe movement similar to that of boiling water. This definition is roughly synonymous with words like roiling, churning, swirling, agitated, bubbling or seething when they are used to describe or define a movement. E.g. the sea boiled in the storm - which means that the sea was rough, not that it was 100C.
Therefore, the phrase boiling in from evokes an image of fiercely agitated, churning swirling motion when talking about the manner of the people coming in from the field. The people weren't just coming in, they were pouring in, there were a lot of people.
I haven't re-read the book for a long time so I'll try to help you towards a personal understanding of the situation rather than give you a clear-cut explanation, which anyway might be debatable, and "primarily opinion-based" questions and answers are not welcome on Stack Exchange sites.
Although the vocabulary used here is particular to Dune (the words heat and boiling are frequent in the novel) it is of the same type as when we say "I was running up the hill" where the preposition (up) expresses a movement of A ("I" in my example) in relation to B ("the hill" in my example) and the verb (running) an action of A.
In describes a movement, boiling describes an action or a state.
means that the men who had been in the fields had begun to arrive in the room when "he" reached the room.
These men were boiling. The use of this word can imply two things.