My shock turns to horror when I see that for the first page of vowel values the author often adds at the end of her description "similar to" followed by an exemplar out of French, German, or English. As she progresses through the values, though, "similar to" becomes "rather like", "approaches", or (my favorite) "vaguely like" and the language, Dutch. So you get stuff like "The first element is a sound between [ö] and [ö:][the goofy 'A's are o-umlauts--they show up just fine in the "dashboard" window] a, followed by a schwa offglide." I leave the second element to your imagination.
The horror turns to terror when I start trying to understand the Frisian pronunciation feature called by my dictionary "breaking". The author devotes half a page to a discussion of the effect of "breaking" on nouns alone. To greatly oversimplify, when you commit any act upon a noun, like, say, attaching a suffix, you are likely but not certain to cause an only partially predictable vowel value change in the accented syllable (which may or may not be reflected in the spelling). Oh please.
Dutch is difficult enough. Some of its sounds could be described: Purse your lips and maintain a light gargle immediately atop the larynx whilst humming, with the lips still open, measure nn of the Queen of the Night's second aria. Then, for Frisian, throw in a few "offglides" and a bit of "breaking".
So much for spoken Frisian although I would still like to hear it.
As it happens, I've taken some lessons on the internet via this site. Ah, crap! Pyt Kramer's course is gone. I could have done with a refresher. Lazy slug wouldn't pay his bill no doubt. Assuming no health problems. Here is a written lecture of his; a statistical study of the vibrancy of his mother tongue.
Good old Pyt! [Remember, that's a germanic 'y', and, being Frisian, make it a diphthong with an off glide. And no fair making it a triphthong! (Though I don't get it either, why a diphthong with an offglide isn't a triphthong. An offglide only counts as half a vowel? Says who?)]
Update: here are the Frisian Vowels.