"Nationalism and Culture" is basically Rocker's response to the rise of the Nazis, which explains, to a certain extent, its idiosyncrasies and occasionally feverish prose (though the latter could be due to the translation, of course). The preface notes that this manuscript was literally the only thing Rocker brought with him when he escaped Nazi Germany (on, Wikipedia informs me, the last train to Basel that was not searched): under these circumstances, it's not too surprising that the work tends to take the form of a polemic, albeit an impressively well-read and often interesting one. The title describes the structure of the book -- loosely speaking, the first half discusses nationalism, the second half culture -- as well as Rocker's fundamental argument: that nationalism and culture are intrinsically opposed. His goal in the book was essentially to refute Nazism by refuting nationalism, but time has rendered the first part of that objective essentially superfluous -- for instance, the chapters where he attacks the variety of racial theories that were current at the time are occasionally interesting but rather dated -- and his arguments on the second head are not always entirely persuasive, largely because of their maximalist nature. To take one example, Rocker spends a considerable amount of time trying to prove his thesis that political centralization is the death of culture by presenting ancient Greece, with its many city-states, as humanity's cultural acme, and the strongly centralized Roman Empire as its cultural nadir. However, his argument consists largely of giving you lists of names of artists, writers, architects (Rocker is big on architecture), and other cultural figures who I had either never heard of, or had heard of but knew little to nothing about, or knew a little bit about but hadn't read in years, and either praising them to the skies (if Greek) or dismissing them as at best second-rate (if Roman). While he may be correct in his judgments, the all-or-nothing nature of the argument, combined with the fact that art is, at least in part, subjective, makes it difficult (for me at least) to believe him without first-hand knowledge of all the people he's talking about. His comparison of Muslim and Christian Spain was even less persuasive, as even he is forced to admit the greatness of, e.g., Goya. I'd be perfectly willing to believe that Muslim Spain and ancient Greece were culturally superior to Christian Spain and ancient Rome due to the fact that the former were less centralized and absolutist than the latter, but trying to cast the latter as cultural wastelands just seems too extreme.
Rocker is on firmer ground when it comes to political philosophy: his analyses of Rousseau, Hegel, and Kant (he doesn't like any of them), the French Revolution (started well but went downhill fast), the Reformation (ditto: he is especially scathing on the topic of John Calvin) were all quite interesting, and his discussion of the rise of the Romantics and German nationalism gives an overview of the kind of ideas that the Nazis built on during their rise to power. Moreover, his explication of the fundamental connection between democracy and nationalism might have been the strongest part of the book. However, it hasn't aged all that well (the almost total lack of attention to anything beyond Europe, with the exception of some quotes from Tagore, is particularly noticeable), and the cultural section, with its tendency to take arguments to extremes, makes the second half of the book a bit of a slog.