\documentclass[10pt]{book}
\usepackage{amsmath}  % removing this bypasses the problem
\begin{document}
\begin{equation} % Using \[ instead is ok
y(n) = \langle x\rangle. % \qquad\hbox{($x$ real)}
\label{eq1}
\end{equation} % Using \] instead is ok
\begin{equation} % Using \[ instead is ok
y_2(n) 
\label{eq2}
= \langle x_2\rangle. % \qquad\hbox{($x$ real)}
\end{equation} % Using \] instead is ok
See eq.~\ref{eq1} and eq.~\ref{eq2}.


%http://tug.org/pipermail/latex2html/2017-September/003974.html
\begin{equation}
% a comment
  \label{test1}
  \int_0^1\sin x dx
\end{equation}
\begin{equation}
  \int_0^2\sin x dx
  \label{test2}
\end{equation}

\begin{align}
a &= a + a + a + a \\
b &= b
\end{align}

\begin{multline}
( a + a + a + a \\
 + b		\\
 + c )
\end{multline}


\end{document}
