It can certainly be said what this is not. It is not nonsense, despite what some may say. No mere whimsy and phantasm can turn a man like him away from the view in the Tractatus. It is not skepticism, as Kripke maintains. (Refuting another philosophy does not make you a skeptic.) Neither is it systemic, like the Tractatus, though it is not a work you can take parts of and ignore the rest, or at least not before understanding it as a whole.

I would refrain from using secondary material to understand this. It certainly could help, but it will paint Wittgenstein inevitably in a flat manner, and discourage you from wrestling with the dimensionality of the book. It is sometimes described as cryptic, but it is not in its nature a work in code like the Phenomenology nor a product like the first Critique of the author's being "too old to devote uninterrupted effort both to completing a work and also to the rounding, smoothing, and lubricating of each of its parts." Wittgenstein knows at every moment what he is saying.

Required reading before: the Tractatus Logico-Philosophicus, himself he says the Investigations can only be properly understood in light of that earlier work. (I don't remember the exact phrasing, but it's in the preface.) You must be clear exactly what the classical view of language entails, before you can understand the weight of the critique against it. At least that was how I experienced it.