A little comment extra-books,

    First what is a plateau? it is a period of little growth or decline; there is another definition that reads "a level of land area considerably raised above adjoining land." but discard this latter for a while.

     So, the learning plateau is the period in which you 'think' you aren´t learning nothing more.

     To illustrate I will give you an example: a pupil can consider her/himself learned in verb TO BE, until the day he/she will find a phrase with GONE, WENT etc. So the pupil here is in the learning plateau, not meaning that he will not learn a bit of information; only he will achieve fewer than usual (when he began the learning of English).

     Other example; we all know the Swiss Army (pen)knife [the primary knowledge we all have, thus we had the "peak" of achievement in the matter]. But what not all of us know is that till 1891 was Germany that supplied this sort of penknives to Swiss Army.

      In 1891, as a threshold in this affair (Swiss Army affair, I mean); Karl Elsener set up the Swiss Guild of knife-making (Grêmio Suíco de cutelaria).

      See? up to day, I knew about the Swiss Army knife and if I would live two hundreds years (not being a Switzerland´s citizen) without this little bit of information; yet I found it interesting anyway.

      So, in my view, the learning plateau is a place where you stand when you can get by, but you may wish learn more and go beyond the simple side of things or go to a more interesting side.

      Sometimes we don´t need climb the peak of knowledge, plateau is enough; but other times it´s matter of life or death.    One example is when the Americans were running from the draft uncle Sam was imposing to youngers, so when they attempted tresspass the US-Canada border they were asked to read a number that happened to be "0", so the real Canadians (or beyond plateau learning in the language) would say Zed, and the others would say Ziro (they never could imagine before they would be in need of a knowledge like this).

      Sorry for being so "type-active" and verbose, but I think I might you got to the point.   So, I hope this helps.