I need to include a XSL in another XSL. Like A.xsl will be containing part of HTML like a side navigation bar, etc and B.xsl will be another with a lot of other HTML transformations. I need to include the result of A.xsl in B.xsl. How can I do this in XSL ?

There is an xsl:include and an xsl:import element.
Look at http://www.w3.org/TR/xslt#section-Combining-Stylesheets