What type of phrase is "Of Unsaid Goodbyes"? Is it grammatically valid for a title of a book?
Specifically, in what way has "of" been used here? Is this a common use of the word?
of is a preposition meaning: relating to.
And what of (= tell me about) young Adrian? How is he? (formal)
(Cambridge Dictionary)
The above usage as well as the one you cite is more literary.
Of is a preposition, that can function as the start of a title, as can on (as in 'On the Subject of...') or about (as in 'About a Boy'). It even stands to reason, that it is in some form an abbreviation of 'On the Subject Of...', though this is speculative. To me as a non-native english speaker it sounds very correct, since in german, the word von is used similarly and mirrors the function of of. Originally titles starting with Of suggested a scientific or at least non-fictional content, but in fact, have since been used more often in fictional contexts. I am still waiting for a title riddiculing this entirely, and using it ironically as in 'Of Farts'.
Titles I quickly found scrolling through ISBN (https://isbndb.com/search/books/Of), starting with 'Of ...':
- Of Courage, of Grief, of Celebration: Making the Most of Your Life in Changing Times
- Of Vicissitudes Of Things
- Of Regimen Of Health
- Of The Serpent, Of Paradise, And Of The Garden Of Eden
- Of Poverty Of Spirit Of Observed In The Midst Of Riches
- Of the plurality of worlds
- Of The Origin Of The World Of Fire
- Of Old:Poems of Time: Poems of Time
- Of Men & Of Angels
On a side note: Apparently, there exist several books simply titled 'Of'.
On another side note: There is a band called 'Of Monsters and Men'.