Lady Charlotte Guest was a daughter of the ninth Earl of Lindsey, and so a member of the English upper class. Born in 1812, she came to Wales as the wife of John Guest, an ironmaster and member of parliament, and while residing there began to work on translating a number of stories from medieval and early modern Welsh manuscripts, eventually published in seven volumes, 1838-1845, as "The Mabinogion." (The title is itself a mistake, but now so embedded in usage that it may never be eradicated.) It was very aristocratic way of involving herself in Welsh culture, and certainly avoided many of the grubby realities, although that may not have been what she intended.
She was not the first translator of some of the stories, and she had assistance, but, considering that she was starting from learning Welsh to begin with, her industry is impressive, and the literary success of her project well deserved -- Tennyson was only one of her admirers. The stories she translated have, with minor variations, become a canonical set: she used pretty much every example of early secular Welsh narrative that wasn't either plainly historical or clearly a straightforward translation from another language. Her version was, and is, widely read, and has helped form a vision of Welsh (and generally Celtic) literature of considerable influence -- not all of it good.
Her husband's death, and her involvement in managing his business, another extraordinary effort for a proper early-Victorian Lady, took Charlotte Guest away from medieval Welsh studies; from the Middle Ages to the Industrial Revolution. After her remarriage to Charles Schreiber (her son's tutor, another interesting story), she turned her energies in a third direction, the collection of ceramics. (To my considerable annoyance, the modern editor of her journals considered this of far, far more importance than either the iron industry or that Welsh literature stuff, and omitted much of both to make room for descriptions of buying fine porcelain ... .) She then went on to become on expert on fans, as well. Much more ladylike, I'm sure; except that she also became an authority on their construction. A really remarkable person, deserving of respect.
This is relevant because, although she lived until 1895, her work on medieval Welsh effectively came to an end a half-century before, and was that of a devoted amateur even then. The translation was not only based on poorly-edited texts, imperfectly understood: in accordance with the practice of the time, she omitted, or at least veiled in obscure phrasing, whatever she found morally offensive, and cut or expanded descriptions, and, generally produced a work that, except on the level of graceful writing, has not stood up well.
Add that early nineteenth-century Welsh studies were plagued by the hand of Iolo Morganwg, who combined real learning with a taste for fraud, and you will realize that what she says may be not only obsolete, but a good-faith repetition of falsified texts, or complete fabrications. (For example, Iolo wrote two sets of Welsh Triads to go along with the real ones; his versions are still being quoted by the unaware, the careless, and those who can't bear to give up a convenient lie.)
However, Charlotte Guest's "Mabinogion" is long out of copyright, which seems to guarantee it a place in some publishers' lists.
Editions which include at least some of the notes she provided still have some value as representing early nineteenth-century knowledge of Welsh literature in the English-speaking world. The 1906 Everyman's Library edition is an example of this approach; a small (mass-market paperback size) hardcover, based on a one-volume edition of 1877, it was almost 450 pages long. In fact, a full, critical edition of her work would probably be of great importance to Victorian studies.
The edition illustrated by Alan Lee reportedly includes Guest's notes; if so, it must be a striking combination of modern fantasy art and useful resource. Stories-only editions of the Guest translation, however inexpensive, are of much less, even dubious, value. The reader may think, "so this is what it is like," when it isn't.
Still, a searchable digital text is an enormous convenience, so long as it isn't mistaken for something definitive, or even very reliable. And a convenient paperback, like the Dover Thrift Edition, is probably a great convenience for readers of Tennyson, and a lot of early twentieth-century Celtic and Arthurian fiction (although even for this, her notes are sometimes of equal or greater importance).
For those with any real interest in the subject, three other translations are available. Gwyn Jones and Thomas Jones translated "The Mabinogion" in the 1940s, and their version replaced Guest's in the Everyman's Library series in 1949. After several revisions, it remains in print, in hardcover (Knopf) and paperback (Everyman Paperback Classics). I have reviewed the paperback edition of this at some length.
More recent is Jeffrey Gantz's "The Mabinogion" for Penguin Classics (1976), which is more modern in language. Like Jones and Jones (and a now-unavailable 1929 translation), Gantz omits one of Guest's selections, "The Tale of Taliesin." He also departs from the usual spelling of some characters' names, for reasons which do not seem entirely clear. As prose, it doesn't seem to me close to Jones and Jones, and well behind Guest, but some people seem to prefer it.
Finally, we come to "The Mabinogi, and Other Medieval Welsh Tales," edited and translated by Patrick K. Ford (1977), which drops the common title, and with it three heavily French-influenced tales, and two other stories with debatable features, but restores "Taliesin," re-edited from manuscripts not subjected to Iolo Morganwg's meddling; in his version, it appears in two parts, "The Tale of Gwion Bach" and "The Tale of Taliesin." He also includes a translation of "Cad Goddeu," or "The Battle of the Trees," an unsatisfactory version of which Charlotte Guest had used in her notes. Ford's introductions are clear and informative; and he was acutely aware of the modern literary uses of the stories. (I know this last because I was taking a course from him while the translation was in draft.) The translation doesn't try to make a medieval text sound more modern than it is, but it doesn't try to make it quaint or archaic, either.
With any of these available to you, Charlotte Guest's translation will make an interesting supplement. One thing it doesn't lack is charm.