You are correct; normally text books only describe what happens in a TIG arc. The problem arises when we try to understand the heat distribution in a SMAW arc, using the TIG arc heat distribution logic. I have heard arguments for example, when welding SMAW, reverse polarity, electrons moving form the weld pool to the consumable electrode, heat are generated by some electrons striking the molten metal being transferred to the pool instead of the electrode itself thus generating heat, resulting in deeper penetration. However I do not have any reference to support this. The AWS handbook, volume 1, explains the higher melt of rate of SMAW, strait polarity, as follows:
Substantially more heat is generated and deposited in the electrode when operating DCEN (excluding tungsten electrodes). Most commercial metals and their alloys formed what is called a cold cathode. Opposite to this are high melting point metals like tungsten and molybdenum. The high melting point tungsten is called thermionic; these metals easily supply electrons to sustain the arc at high temperatures. On the other hand, cold cathode low melting point metals create great quantities of energy (heat) during the release of electrons to support the arc. This happens because electrons do not easily escapes the surface of these metals. This explains the higher deposition rates and electrode melting rate for SMAW, strait polarity.

Unfortunately they do not describe what happens during SMAW reverse polarity, just changing the above mechanism help explaining more heat at the work but applying it to TIG welding runs into difficulty when trying to explain the shallow penetration when welding reverse polarity TIG.

Hopefully somebody can give more info to clarify this

Joe

Some texts explain that it has to do with the dynamics of the arc and more importantly the gas ions from smaw and the pinch effect of the electrode covering and arc. In general just accept it since welding tig with dcep is not practical