Is memoir class like an extended version of book class?

Yes, memoir is ideal for a humanities thesis. I am using it for mine.

The memoir class incorporates the functions of a large number of commonly used packages to provide a more consistent interface, and is designed to be easy to customize. So instead of loading a dozen possibly conflicting packages---geometry to change margins, and setspace to change line spacing, and titlesec to format section headings, etc., etc.---you just specify memoir as your document class and use its settings, all lovingly detailed in the documentation at texdoc memoir. There's even an example of how to set up an academic thesis in the manual.

From the memoir manual:

The memoir class is an attempt to integrate some of the more design-related packages with the LaTeX book class.[...] The memoir class effectively incorporates the facilities that are usually accessed by using external packages. In most cases the class code is new code re-implementing package functionalities.

IMHO is a good document class to make a thesis, or any other document ... but there many good thesis templates using different documents classes. Specially if you are newbie, take your time to find the template\document class with a "default output" most near to your requirements, so you will need a minimal customization. Good starting points are LaTeX Templates, writeLaTeX and, of course CTAN.