"Books that I really enjoy dipping-into but never read all the way through" seems to be a definite category in my lifetime.
I first became aware of this with Robert Burton's Anatomy of Melancholy (published from 1621 onwards); which I used to keep by me in my twenties, but never read consecutively, and probably only a modest proportion of its total. Indeed, I would probably have named it as my desert island book for exactly the reason that I would be unlikely ever to finish it, yet enjoyed returning again and again.
Another, albeit shorter, book of this type I engaged-with for a few years was Hugh MacDiarmid's autobiography Lucky Poet (1943); which I would similarly browse almost randomly in; and stumble across passages that I did not recognize but appreciated - and then could never find again!
This business of going exploring then getting lost in a book, and stumbling across what seem like new treasures (but which may have been read before, in a less appreciative mood); is one of the pleasures of this kind of reading - and for this reason it is better when the book is both big and dense or complex in arrangement (and when it lacks a good index!).
Perhaps the earliest example was our family Complete Shakespeare in a WWII economy-paper edition - with woodchips; which had been salvaged by my father from an army library being discarded in occupied Germany circa 1950. As a young child, I used to pick this up and read the titles, and passages here and there - with excited incomprehension. As a result I was far more impressed by The Phoenix and the Turtle, All's Well That Ends Well, or Love's Labours Lost - because of their intriguing names; than the lists of 'King X' plays, or the single word titles like Sonnets, Macbeth or Hamlet.
Nowadays; my undisputed champion dipper-in and desert island book is The History of Middle Earth (in 12 or 13 volumes according to definition) edited by Christopher Tolkien. I have read-in this book for hundreds of hours; never get sick of it, keep discovering new surprises and pleasure - yet never seem to get any closer to completing or comprehending it.
Not finishing favourite books is a much under-rated pastime.