Why does concentration of solutions of alkali metals in liquid ammonia change the magnetic properties?
I always suggest students to try Google Scholar (scholar.google.com) when a simple Google search fails. I just searched three keywords : alkali metals ammonia solutions and the third result is highly relevant.

When your book talks about "in concentrated solution", it means more alkali metal in liquid ammonia. This paper, which you should search in Google Scholar, is titled A Revised Model for Ammonia Solutions of Alkali Metals, Journal of the American Society, Year 1962, volume 84, pg 2264). It talks about the formation of dimers of metals of the type $\ce{M2}$, because as you increase the metal conc. in ammonia solution conductivity decreases.
The authors write, raising objections to older models as completely wrong show the following evidence:


  "We picture the » species as a quadrupolar ionic assembly of $\ce{e- + 2M+}$ in which there is little distortion of either the ammoniated electrons or ammoniated metal ions. Presumably the electrons and ions are held in a square or rhombic configuration."


In short the picture is not that simple an no wonder the authors of a grade 12 book never discuss more details. You can start from here and search more.