Books can be attributed to "Unknown" when the author or editor (as applicable) is not known and cannot be discovered. If at all possible, list at least one actual author or editor for a book instead of using "Unknown".
Books whose authorship is purposefully withheld should be attributed instead to Anonymous.
This is where I learned such things as that there are no "k"s in written Welsh because there wouldn't have been enough of them in a typical set of "moveable type" used for English and what dipthongs and tripthongs are.
This is a well written and useful dictionary that includes a bit of history about the Welsh language as well as guidance on pronunciation, abbreviations and personal and place names.
I was gifted this by my Welsh grandmother. It is a straightforward and clear dictionary, a pleasant size to use, clear type, and uncomplicated (unlike to Welsh language ...). Recommended, even though the word-set is now out of date.