This book is important because it lays out a minor strand of British materialist thought which was vitalistic and not deterministic, but which must have been familiar to Marx, and which I believe actually influenced Marx.
According to this view, matter is not dead and immobile, but is vital and capable of thought and action. Here is the history of this idea:
The early Presocratics adopted an activist view of matter (hylozoism). Plato does not have a theory of matter, the three components of reality being (for him) space, soul (self-motion), and the Ideas. Aristotle's view of matter, on the other hand, is quite complex: matter, in Aristotle, is separated from the cause of motion which ultimately resides in God (the Prime Mover). The Prime Mover is pure form and is thus immobile, while matter itself has an "urge" -- it seeks form like a woman seeks a man (Aristotle's image; not mine). But from this conception of matter as distinct from the cause of matter (which is forever external to it) developed the notion of matter as passive and dead, as also the notion that all rectilineal motion must have an external cause.
Epicurus, though a materialist, rejected a purely mechanistic conception of matter. According to Epicurus, everything consists of innumerable atoms swirling in an infinite void. The natural motion of atoms is downward motion. Thus, given sufficient time, all the atoms would end up simply raining downwards. Since this would involve the dissolution of worlds, and as worlds have not dissolved -- there must be an ever-so-slight inclination or swerve of the atom -- an intrinsic, spontaneous motion -- that prevents this from occurring -- in other words, the atom is capable of self-initiated motion. From this, Epicurus inferred that "man is free" -- that is, that human action is not entirely deterministic.
There is some controversy on this point, as the extant fragments of Epicurus do not mention the swerve, and it is only the testimony of Lurcretius and Cicero that it can be ascribed to Epicurus. But modern scholarship is virtually unanimous that Epicurus held this doctrine.
Marx did his University dissertation on Epicurus and the swerve of the atom -- and arguing (on purely philological grounds) against the deterministic interpretation of Epicurus championed by Gassendi, claimed (rightly) that Epicurus did indeed hold to this theory of the swerve. And from this Marx, too, inferred that: ergo, Man is Free! (the "!" being Marx').
This vitalistic conception of matter can be found in the Manuscripts of 1844 (where Marx criticizes a purely mechanistic conception of materialism common to the Brits), and it is also implicit (in my opinion) in the German Ideology. It is not discussed, but can be assumed in Kapital I. At any rate, it is the metaphysical supposition on which Mondolfo's humanistic reading of Marx is predicated.
While the source of Marx' view undoubtedly lay in Greek philosophy, the presence of this vitalistic strand in early and mid-19th cen. British materialism suggests that Marx was probably influenced by writers like Priestley (http://en.wikipedia.org/wiki/Joseph_Priestley), one of the principal figures in Yolton's book.
There is an excellent appreciation of this aspect of Marxism in A. James Gregor's book on Giovanni Gentile:
Gregor finds this aspect of Marxism a contradiction in terms, and is therefore critical of it -- preferring an idealistic conception of motion, derived from Gentile's Actualism, which is a form of transcendental subjectivism.
At any rate, this is the aspect of Marx' thought that neither Engels nor Lenin understood -- and their error has had quite tragic consequences.
Yolton was a student of Julius Weinberg -- and was not nearly as brilliant as Weinberg. This book is good, but not outstanding -- but its topic is of great importance, as I have here tried to show.
The above, of course, is only my layman's understanding of the matter (no pun intended) -- as it has been 30 years since I have studied this material. So take it for what little, I'm sure, it's worth....