There is a small technical error in the "Heat Transfer" section of the amateur rocket designer's text. At the end of this section the author advises that:
Material failure is usually caused by either raising the wall temperature on the gas side so as to weaken, melt, or damage the wall material or by raising the wall temperature on the liquid coolant side so as to vaporize the liquid next to the wall. The consequent failure is caused because of the sharp temperature rise in the wall caused by exessive heat transfer to the boiling coolant.
Boiling is usually a good thing from the standpoint of heat transfer. The author has it right that boiling usually increases the heat transfer. Increased heat transfer, however, results in lowered temperature. Think about that for a minute and it will become obvious.
The problem with boiling in this application results from something that is called alternately film boiling, jacket boiling, or Liedenfrost boiling. The boiling rate becomes so intense that a layer of vapor is established between the hot wall and the cool liquid. The heat transfer properties of boiling liquid are really wonderful. The heat transfer properties of gaseous vapor really suck. Once the vapor barrier is established, whatever it is that is being cooled will likely melt.