Change line spacing inside the document

You can use \setstretch{}. If you want to only affect a certain content you can use it with a group.

You can also apply any size changing switches such as \small or \tiny inside the {} group as well.

Code

\documentclass[12pt,a4paper]{book} 
\usepackage{xcolor}
\usepackage{lipsum}
\usepackage{setspace}

\renewcommand{\baselinestretch}{1.5} 

\begin{document}
\lipsum[1]
{\setstretch{1.0}\color{blue}
\lipsum[2]
}
\lipsum[3]
\end{document}

Alternatively, the following solution is a bit cleaner:

\documentclass[12pt,a4paper]{book} 
\usepackage{lipsum}
\usepackage{setspace}

\begin{document}

\onehalfspacing % Set line spacing to 1.5

\lipsum[1]

\singlespacing % Reset line spacing to 1 from here on
\lipsum[2]
\onehalfspacing % Reset line spacing to 1.5 from here on

\lipsum[3]
\end{document}

Also, instead of \singlespacing and \onehalfspacing you can use \setstretch{1} and \setstretch{1.5} for greater flexibility.

Other values

Other line spacing values that lie in between onehalf, single and double can also be achieved with:

\usepackage{setspace}

\begin{spacing}{1.125}
…
\end{spacing}