I don't want to pretend to know the history of linguistics better than I do. Supposedly, Trubeckoj's definition of the phoneme was foundational for modern phonology. This seems to me to be a sort of in-the-air phenomenon… phonemenon? Various other Prague types had been attempting definitions, & the core of the Trubeckojan phoneme is implicit in a reasonable reading of Saussure. However, to take at least Karl Bühler's contemporary reading, Trubeckoj's conceptualisation of the phoneme opened up for him a phonology liberated from phonetics.
But the concept of a phoneme is simple enough, & one doesn't need to read 280 pages of phonological theory to understand it. The most exciting parts of this book, it seems to me, are the first three chapters, in which T—I'm summarising from my own bias, here—takes language as a linear system of signs based in difference, & inductively theorises the structural possibilities for differentiation.
Chapter IV applies that inductive system to the analysis of the phonetic systems of dozens of languages. This chapter is the longest of the book, & it can be slow going. It is generally persuasive. However, T seems to fail to stick to structural principles, & allows phonetic considerations to interfere—as in, for example, his distinction between consonants & vowels. T is aiming at getting at linguistic universals, & this may be part of what leads him toward phonetic explanatory expediency, when structural consistency would be better supported by his overall arguments. Additionally, T uses terminology that is not standard today (there was no standard in his time), & sometimes it can be difficult to figure out what he means by a particular term if you're not familiar with the particular language he's describing. Finally, I caught a couple errors for languages I know. Nonetheless, if you're interested in structuralism in a specifically linguistic context, the chapter is worth while.
The subsequent chapters are brief, quick reads, & are perhaps because of this disappointing, but suggestive. They handle some cool aspects of speech sound analysis beyond the phoneme. This should be suggestive for how other structuralist efforts might look beyond the basic unit.
If you're *not* interested in the particularly linguistic aspects of the history of structuralism, chapters I–III are still worthwhile. Chapter IV might be a good one to skim. The remainder of the book can be handled quickly.