While this is an accessible, fairly broad introduction to Celtic myths, this text lacks commentary and is poorly edited, making it a mediocre introduction at best. The Encylopedia of Celtic Myth and Legend contains a decent range of myths, grouped by theme, with a short introduction to each myth. The range of stories is fairly broad and contains a number of important, highly referenced tales. However, the book obviously doesn't contain all of the myths, including some important stories. The theme grouping can make it difficult to keep characters and time lines straight, but it is historically accurate. The introductions to the myths are limited, often informal, and while they do give a sometimes useful plot overview, don't provide much context or information about the myth or its setting within the Celtic mythos. The myths themselves contain no footnotes, minimal name translations, and are poorly edited (there's a lot of missing punctuation and a few typos). The lack of commentary and information makes the myths less accessible to the reader: character identification is difficult, plot cycles and timelines are confused, and cultural notes and explanations are lost. On the whole, this is a mediocre introduction: the myths are there, but they are hard to interpret and not very useful to the reader. However, this book does appear to be easier to find at local libraries than longer, more annotated Celtic mythology collections, so it may be a good introduction to a reader curious about the myths but unwilling to buy a book without reading more. I recommend it only on that basis: borrow it and read it for an idea of Celtic mythology, but purchase a different book for in depth study.
Without a doubt, the Celtic myths themselves are complex, enjoyable, epic, and interesting. The scope of genres, from invasions to births to cattle raids to voyages to adventures to deaths (and many more) allow for stories on any subject and for any occasion. Some of the characters are gods, some demi-gods, some epic heroes, and the events are all compelling and mythic. Almost everything, from battles to feats to love, happens on an exaggerated scale, but the exaggeration renders the emotions and characters more vibrant, making them hyper-realistic rather than unrealistic or difficult to connect with. Overall, the myths themselves can't be faulted. The texts can be repetitive, either because of style or because of the way that tales were written and transcribed, but at its best the language is descriptive and invokes interest and emotion in the reader. If Celtic myths interest you, this text can serve as an introduction to the characters, stories, and style that makes up Celtic mythology, and it may therefore be a useful resource.
While the myths can't be faulted, there are a number of problems with the Matthews's editing, compiling, and annotation. The editing is shoddy, and the missing punctuation and spelling errors are both frustrating and confusing--because of the writing style and Gaelic phrases, it's sometimes difficult to determine what are errors and what are meaningful, original inclusions and words. The myths in this compilation are fairly broad but by no means all encompassing, and the editors often reference other texts (usually their own) for more myths and stories about the characters. Finally, the dearth of annotation and information about the myths keeps this compilation from being truly useful. There is introductory information that mentions characters, a little bit of context, and the manuscript the myth is based on/take from, but these introductions are short, informal, and don't provide nearly enough information about time line or characters. The myths themselves are completely unannotated, which makes for smoother reading but doesn't provide the reader any information about details, connections, or explanations that they might be missing. As a result, this complication is good only for some basic sampling of the Celtic myths--as an in depth resource and source of information, it is pretty much worthless.
If Celtic mythology interests you but you don't know much about it and want a quick introduction to the myths themselves, consider looking for this book at a local library. To my knowledge, it is fairly easy to find locally while other, better books of Celtic myth may only be available if ordered or bought online. As an introduction, this book provides an overview of the style and stories of Celtic myth. However, I don't recommend buying this book or using it as a resource. Between the shoddy editing, missing stories, and lack of additional information, it isn't worth owning.