This is probably a good starting point for someone who doesn't know anything yet beyond the 19th century--knowledge of which is kinda assumed. (if you haven't read Marx, Hegel, Kant, et al., why mess with this stuff?)
It's organized around three schools of thought. It could be organized differently--Derrida is in the phenomenology section but is often thought of for his interaction with structuralism, just as Foucault is presented as part of structuralism but came out of phenomenology. Marcuse is a heideggerian, whereas Lukacs and Gramsci have little association with critical theory stricto sensu, which seems more directly associated with Frankfurt. These are but quibbles, however--the text reads easily and gives some perspective. Very much a text for students.