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}
The following does what is wanted, for tabulars. It uses the fact that tabular line spacing is determined by \strutbox whose height and depth in the memoir class are defined to be respectively .7 and .3 of the current \baselineskip.
Actually, this is the same as in the article class. Anyhow using \strutbox in the code is better, so I updated accordingly the answer. And the code now also takes into account \arraystretch (changes to \arraystretch should happen before the \MySpacing environment).
The included output has examples on two columns.
Last edit: new version of the helper macro \AfterMySpacing, which deals better with a page break occuring exactly after the end of the \MySpacing environment.
Added explanations: this document (with memoir class) starts in \OnehalfSpacing mode (this is a macro of memoir which enlarges the \baselineskip by a value depending on the type size, for 10pt the factor is 1.25). Then the column on the right switches to \DoubleSpacing (this does not mean that the interline spacing is doubled, the ratio is rather 1.667 for 10pt size).
The boxed values are the parameters given to the \MySpacing environment. This parameter tells the environment which multiple of the standard baselineskip (whose value is set up by the class depending on the font size option passed to it) to use for interline spacing in the tabular environment (recall that tabulars add \struts to ensure regular line spacing -- a safeguard which fails if a cell contents has a height or depth larger than the used strut). This multiplier is applied to the default \baselineskip, not to the one used in the surrounding paragraph already stretched out either by \OnehalfSpacing or \DoubleSpacing.
Finally, there is a LaTeX parameter called \arraystretch, normally set to 1, which multiplies the interline-spacing of all tabulars and arrays. The environment \MySpacing was written to take it into account. So the examples end with a test of this by setting the \arraystretch to 2 but using parameter .5 so that the net effect is like using \MySpacing with parameter 1.
Note 1: with the parameter given to \MySpacing set to only .75, the strut used in the tabular has (with the default fonts) its height a tiny bit smaller than the height of the letter X (circa half a point smaller), hence the line spacing is not exactly as intended (and can be irregular inside the tabular, depending on its content), although this is not so visible to the naked eye. But this problem goes away for parameter values higher than .82.
Note 2: if the environment \MySpacing contains, rather than a tabular, text in lines (ending with \\ except the last one), the wished-for spacing with respect to the surrounding lines will be obtained by inserting a \strut in both the first and the last lines of the enclosed block. (assuming \arraystretch equal to 1).
\documentclass{memoir}
\usepackage{multicol}
\OnehalfSpacing
\raggedbottom
\makeatletter
% helper macro for the environment MySpacing
% initial version:
%\newcommand{\AfterMySpacing}{\noindent
% \vphantom{\vrule height \baselineskip depth \z@}}
% new method, better if pagebreak occurs right after the MySpacing block
\newcommand{\AfterMySpacing}{%
\vskip\dp\strutbox
\prevdepth\dp\strutbox
\noindent\strut} % <- the \strut here could be removed, TeX adds the
% necessary vertical skip.
% And, actually, perhaps the macro would be more flexible
% also with the ending \noindent removed.
\newenvironment{MySpacing}[1]
{\par\nointerlineskip
\vskip\baselineskip
\def\baselinestretch{#1}\@currsize
\vskip-\arraystretch\ht\strutbox\relax % replaces \vskip-.7\baselineskip
}
{\par\nointerlineskip
\vskip-\arraystretch\dp\strutbox\relax % replaces \vskip-.3\baselineskip
\ignorespacesafterend\aftergroup\AfterMySpacing}
\makeatother
\begin{document}
\begin{multicols}{2}
\fbox{2} Before the block Before the block Before the block\dots
\begin{MySpacing}{2}
\hspace*{1cm}%
\begin{tabular}{ll}
ooo & xxx \\
baz & qux \\
foo & pqr \\
\end{tabular}
\end{MySpacing}
\ldots{} after the block after the block after the block after the block
after the block
\fbox{1} Before the block Before the block Before the block\dots
\begin{MySpacing}{1}
\hspace*{1cm}%
\begin{tabular}{ll}
ooo & XXX \\
baz & qux \\
foo & abc \\
\end{tabular}
\end{MySpacing}
\ldots{} after the block after the block after the block after the block
after the block
\fbox{.75} Before the block Before the block Before the block\dots
\begin{MySpacing}{.75}
\hspace*{1cm}%
\begin{tabular}{ll}
ooo & XXX \\
baz & qux \\
foo & pqr \\
\end{tabular}
\end{MySpacing}
\ldots{} after the block after the block after the block after the block
after the block
\columnbreak
\DoubleSpacing
\verb|\DoubleSpacing|
\fbox{2} Before the block Before the block Before the block\dots
\begin{MySpacing}{2}
\hspace*{1cm}%
\begin{tabular}{ll}
ooo & XXX \\
baz & qux \\
foo & pqr \\
\end{tabular}
\end{MySpacing}
\ldots{} after the block after the block after the block after the block
after the block
\renewcommand{\arraystretch}{2}
\verb|\renewcommand{\arraystretch}{2}|
% for testing purposes
\fbox{.5} Before the block Before the block Before the block\dots
\begin{MySpacing}{.5}
\hspace*{1cm}%
\begin{tabular}{ll}
ooo & XXX \\
baz & qux \\
foo & pqr \\
\end{tabular}
\end{MySpacing}
\ldots{} after the block after the block after the block after the block
after the block
\end{multicols}
\end{document}
Best Answer
Thanks to Ulrike Fischers comment, I figured it out. I had a longtabu in the same section after the text that had singlespacing activated. After changing the code, the spacing was fine again. Here is the longtabu code that caused the issue:
The \singlespacing in the second line is what caused the unexpected behavior, since it was in the same paragraph.