Although the Matthews' have been criticized by scholars for their inaccurate portrayal of Celtic history, this book is relatively immune to such criticism, as it is mainly a poetic work. It doesn't claim to be a scholarly reconstruction of Celtic devotions, but rather is a set of devotions with a Celtic flavor. I've been using it now for about 4 months, and would highly recommend it.
The book is extremely well organized and stuffs a lot of information into a very slim book. It is organized into four sections - one for each of the four seasons - Samhain, Imbolc, Beltane, and Lughnassah (each "season" is said to begin on the first of the month of the festival). For each season, there are seven days of morning and evening devotions. Each devotion has guided meditations (called solar and lunar meditations, done in the morning and at night, respectively), and an unstructured meditation/quiet prayer time (she refers to this as "sacred silence"). The evening devotions also include a self-clarification to look back on your day and reflect on what you've done.
The book is very ritualistic; following its schedule means you do the same set of daily devotions for each week during the season, and the same set of lunar and solar meditations for each month. So you'll do the set of weekly devotions approximately eight times before you move to the next season's devotions, and you'll do each guided meditation approximately twice before you move to the next season's meditations. Personally, I find this both variable and "set" enough to be useful. Also, the devotions can be stretched or shortened to fit the available time by meditating for shorter or longer periods.
Additionally, there are "suggested activities" for each season, calls of welcome and goodbye to each season, and an invocation for each season. The back of the book has some specific prayers and blessings for life events such as birth, death, marriage, etc.
Matthews implies at the beginning that the book is written to work for Celtic Pagans, Celtic Christians, and polytheists. And, that may be true, if you're willing to tweak her prayers a bit. Deities (or the Divine, depending on one's world view) are not called by particular names, but rather by epithets, such as "Lady of Assemblies", or "World-Turner", or "Foster Mother". The devotions also include sections that address one's "soul friends", which can be used to refer to either patron deities or nature or household spirits.
Within my particular world view, her naming approach mostly works, although I sometimes find that I can't obviously match a particular Celtic deity to the epithets, leaving me unsure of who I'm addressing. I also haven't figured out how I feel about randomly addressing a variety of deities (even within my chosen pantheon) without doing an "introduction" ritual first. In any case, I think her approach deftly dodges some of the land mines of religious world views and one can easily modify the calls to deities without doing violence to her poetry.
And finally, the book is beautiful. Each season has an illuminated plate to go with it, and each devotion page has attractive Celtic knot work shadow printed in the background.