This book is incredible. First published in 1934, it is the definitive reference work on the subject. What is a particle, anyway? After acknowledging the difficulty of a definition, Denniston writes: "I will define it as a word expressing a mode of thought, considered either in isolation or in relation to another thought (footnote omitted), or a mood of emotion." An elaborate footnote then makes a number of qualifications, illustrating the impossibility of an airtight definition.
Here's my definition: "Greek particles are little words used alone or in combination with other particles; they possess a wide range of possible meanings that can vary dramatically depending upon the context in which they appear; no matter how many Greek texts you have read, they will continue to drive you insane no matter how hard you try to nail down their exact sense in a given passage -- especially in Homer, Pindar, the lyric poets, Aeschylus, Sophocles, Euripides, Plato, Aristophanes, Xenophon, Herodotus, and any other Greek author you can think of."
This is why Denniston's magisterial treatise is such a helpful aid. If you are an advanced student of classical (Homeric and Attic) Greek, this work is as crucial to your library as Smyth's grammar. It is mind boggling how Denniston could actually have written this work; only a person with an encyclopedic knowledge of classical Greek literature and the ability to carefully sift through every line of it could even conceive of such a herculean task.
Particles are not encountered in NT Greek with anywhere near the frequency of Attic and Homeric (indeed, there are no NT authors referenced in this work), so if your field of study is limited to Koine, you won't need this book.