This is a description of how modern Icelandic evolved out of Old Norse, and it is packed with far more detail than one would expect from its mere seventy pages of text. Stefán Karlsson originally wrote it in Icelandic for Icelanders, but Rory McTurk saw its value for foreign students of Germanic linguistics and philology and produced a fine English translation. Karlsson not only sets out how modern Icelandic is different from Old Norse in phonology, morphology, and lexicon, he also gives dates for all the developments along the way. Indeed, there are many books and webpages out there on modern Icelandic versus Old Norse, but this was the only reference I found on all the intermediate stages of the language, whether the fourteenth century or the eighteenth. Highly recommended as a follow-up to any Old Norse textbook.
Useful short overview of Icelandic. I was particularly interested in the history/social history of the language, printing, literature &c., all of which are covered and set in context. (I skimmed over the more technical linguistic bits, as I'm not planning to learn Icelandic - not at the moment, anyway!). There are some interesting parallels with Welsh: Icelandic was considered to be at risk due to foreign influences at one time, apparently, but it seems to have survived this far rather better than Welsh has. Some lessons for us in Wales! and a good general introduction for anyone interested in Iceland and its culture.