How to calculate time domain SNR using known sequence
The peak of the cross correlation should be the transmit signal power times the channel's attenuation.
Realizing that, it's really just
\begin{align}
\text{SNR} &amp;= \frac{P_\text{signal}}{P_\text{noise}}\\
&amp;= \frac{P_\text{signal}}{P_\text{received} - P_\text{signal}}\\
&amp;= \frac{P_\text{tx}\cdot a_\text{channel}}{P_\text{received} - P_\text{tx}\cdot a_\text{channel}}\\
&amp;= \frac{P_{crosscorr,max}}{P_\text{received}-P_{crosscorr,max}},
\end{align}
which is
=Rxy(tm)^2 / (Py-Rxy(tm)^2)

in your notation.
So, either that book is wrong or you're missing something there.