There are some books I absolutely buy (and I have a rather large library myself, entirely physical), but there are many cases where borrowing makes more sense.
I am a creature of comfort after all!
I use bookmarks, often the paper receipt from the purchase of the book itself.
But it's a great option if you have a modern device.
As you should, but my comment takes "book" to mean the cultural object, encompassing non-physical (e-)books.
This sounds unnecessarily reductive. By "own" I would mean that I can re-read the book again and again and again as many times as I want as long as I take good care of the book and prevent it from disintegrating.
But the DRM e-books can't be used like that. That was their point.