Desultory is too often used pejoratively, as though a diagnosis of attention deficit disorder were pending. We can read it that way, no question, but not always. The word is rooted in the Latin dēsultor, meaning leaper or vaulter (OED), which has entered English furtively as a noun and means “circus horse-leaper.” That implies grace, discipline, strength. Read the first OED definition of desultory and see how it mutates: “Skipping about, jumping or flitting from one thing to another; irregularly shifting, devious; wavering, unsteady.” A butterfly flits, purposefully. Watch one moving from flower to flower, probing for nectar. Butterflies have evolved a proboscis resembling a coiled, retractable straw. Their flitting is methodical, not superficial.
Flitting among books is what some of us do every day. Sometimes the flitting is pragmatic – consulting a dictionary or other reference work. More often, we have several books going simultaneously, and one book inevitably leads to others. I had an obsessive friend who couldn’t read without tracking down every new word or allusion as he encountered. He was a very slow but knowledgeable reader. It’s Borgesian whimsy to imagine all books ultimately meshing into one big book.
The passage quoted at the top is from Edward Gibbon’s posthumously published Memoirs of My Life and Writings (1796), and refers to his time at Westminster School as a boy. “Free desultory reading” is not a failing but an ideal to strive after.