The frequent arbitrariness of national borders in Eastern Europe goes without saying, and just as contentious is where to draw a line between the various languages (or is X "just a dialect" of Y?). Emigration has brought various Slavic languages to Western Europe and Israel with interesting sociolinguistic results. These issues are the subject of the 24 papers in this 2015 collection.
"Handbook" isn't really an appropriate title for this book. It isn't a handy reference limiting itself to the scholarly consensus with each author writing to the same template. Rather, it is an uneven collection of papers, some of which offer original work in the field. The writing styles differ drastically, with e.g. the paper on Croatian by Peti-Stantic & Langston citing Pierre Bourdieu and arguing at length on general sociological theory, while other papers prefer to keep things concise and focused just on their limited area. Most objectionable in calling this a "Handbook" is the inclusion of a paper by Paul Wexler. This scholar has a peculiar view of the origin of the Yiddish language that virtually no one else in the field agrees with (some even question if Wexler himself really believes what he writes). Wexler's paper here is mainly just a space for him to reiterate that theory instead of dealing with specifically Slavic themes like the book title suggests, and he extends his views of ethnogenesis to the Romani language as well, with equally unsatisfying results.
Still, anyone interested in the Slavic languages overall will find much to enjoy. There are papers on the most famous Slavic minorities like the Rusyns (Michael Moser) and Sorbs (Roland Marti), but we also get contributions on minorities that are likely to still seem very exotic to most readers like the Burgenland Croats (E. Wayles Browne), the Banat Bulgarians (Motoko Nomachi), and Gorani of eastern Albania (Klaus Steinke). I had no idea that the Polish language once had a significant presence in the Latvian region of Latgalia until I read Catherine Gibson's paper here.
The lack of contributions on issues of language, borders, and identity within Russia weakens this collection as a pan-Slavic overview. (Something could be said about recent work on resurrection of Pomor identity or Siberian issues.) The Russian language is discussed here, but only among diasporas in Israel and Ireland.