Giles, J. R. 1987. Introduction to the Analysis of Metric Spaces. Vol. Australian Mathematical Society lecture series. Cambridge: Cambridge University Press.
Kreyszig, Erwin. 1989. Introductory Functional Analysis with Applications. Wiley Classics Library ed. Vol. Wiley classics library. New York: Wiley.
Young, Nicholas. 1988. An Introduction to Hilbert Space. Cambridge: Cambridge University Press. https://eu.alma.exlibrisgroup.com/view/action/uresolver.do?operation=resolveService&package_service_id=5029053650002418&institutionId=2418&customerId=2415.
