Is the ethyl cation really more stable than benzylic and allylic carbocations?
I believe this to be a simple typo in the book. In the original image you'll see that in the surrounding schemes $&lt;$ and $\to\text{Stability}$ are given. Only in this line the relations are reversed, while the stability indicator remains the same.

However, since we're at it, we can at least try to put some values to the task. I'll adopt the same scheme used in the comparison of the t-butyl cation and the benzyl cation, or the radicals. I calculated the isodesmic reactions of the form in $\eqref{isodesmic}$ at the DF-B97D3/def2-TZVPP level of theory.
$$\ce{ R+ + CH4 -&gt; RH + H3C+ }\tag{1}\label{isodesmic}$$

According to this we'll find the following order in decreasing stability: t-butyl, benzyl, allyl, (ethyl). Note that the ethyl cation is a non-classical cation, basically a proton coordinating to the &pi;-bond of ethene; see also: Which carbocation is more stable, the ethyl- or 1-propyl-carbocation?.

\begin{array}{llr}
  \ce{R+}  &amp;  \ce{RH}  &amp;  \Delta G / \pu{kJ mol-1}\\\hline
  \ce{H3C+}           &amp;  \ce{CH4}         &amp;      0.0  \\
  \ce{[H2C=CH2]H+}    &amp;  \ce{H3C-CH3}     &amp;   -197.9  \\
  \ce{H2C=CH-CH2+}    &amp;  \ce{H3C-CH=CH2}  &amp;   -259.3  \\
  \ce{H5C6-CH2+}      &amp;  \ce{H2C-C6H5}    &amp;   -343.0  \\
  \ce{(H3C)3C+}       &amp;  \ce{HC(CH3)3}    &amp;   -370.9 \\\hline
\end{array}

(Side note: I was unable to remove a small imaginary mode from the isopropyl carbocation. That has almost no influence though.)

    
     
     
 
      

(I won't attach geometries or absolute energies this time, because that would exceed the character limit.)