Terry Castle, her book jacket informs us, is not only Walter A Haas professor in the humanities at Stanford University but also a miniature dachshund enthusiast. This juxtaposition  so highbrow! So absurd!  is characteristic of her style: at once learned and playful, she is, as that "miniature" promises, a veritable mistress in the art of cutting down to size.

She's best known, on this side of the Atlantic at least, for her London Review of Books obituary of her friend and mentor Susan Sontag. Included in this clutch of 21st-century essays, "Desperately Seeking Susan" remains a glorious exercise in iconoclasm. Among Castle's cherishably disloyal revelations is the scene in which the high priestess of hard thought, clad in white tennis shoes that "made her feet seem comically huge, like Bugs Bunny's", demonstrates what it's like to dodge sniper fire in Bosnia by racing through a tourist-thronged Palo Alto in a hail of imaginary bullets. It's hard to read Regarding the Pain of Others with quite the same reverence after that.

If the spleen only went one way this sort of thing would be vindictive, even nasty, but Castle is too self-aware a critic to carry out anything so brutish as a hatchet job. She persistently casts herself in ridiculous, demeaning roles ("I, her forty-something slave girl from San Francisco"), yelping at one point: "Caveat lector: Lilliputian on the rampage!" The self-belittling reaches its apotheosis in the most substantial essay here, "The Professor", an extraordinarily gleeful account of Castle's damaging relationship with a much older woman when she was a grad student in the midwest.

In this real-life retelling of Bluebeard, The Professor, complete with "the Very Weird Long Grey Braid; the Withered Leg, the Loaded Pistol in the Bedside Drawer; the Room in Her House One Was Never Allowed to Enter", is pitted against the youthful "T-Ball", a naive and horribly intellectually ambitious baby dyke. The result, predictably enough, is carnage, albeit of a kind that anyone who's ever loved and lost might experience considerable cathartic pleasure to encounter.

During the final throes of her healing process, Castle visits a psychiatrist who repeatedly, apparently absent-mindedly, brings up the story of a lesbian who experienced sexual gratification by dressing as an infant, a recollection that inevitably prompts her to muse to the infuriated Terry: "Dey vore dia-peuhs! Dia-peuhs! Dey akchulally vore dia-peuhs!" Even without this mildly disturbing nudge, it's evident that many of the essays here involve confrontation with a Mommy Dearest of one sort or another. Happily for the voyeuristic, Castle's real mother elbows her way through the surrogates to take centre stage in "Travels with My Mother", a sparky, touching account of the shifting power relations between reluctant daughter and ageing, insufferable parent.

Elsewhere, Castle's fine mind and roving eye take in the jazz saxophonist Art Pepper; the relative merits of Georgia O'Keeffe and Agnes Martin ("There is no doubt that Martin's work will always be caviar  the very palest of fish roe  to the general. Who better, then, to serve as my guardian angel?"); and the delights of a holiday in Sicily in which she acquires food poisoning and is forced to defecate into the sea ("Byronic exaltation this is something I have never done before; I am breaking every law of God and Man"). One could rejoice in these essays for many things: their waywardness, their galumphing, wicked humour (Wally the dachshund is "as slutty and insouciant as Private Lynndie England. All she needs is a dangling cigarette and a tiny pair of four-legged camouflage pants"). But these are surface pleasures, flung up on what seems a deeper quest to burrow into a subject, any subject, and make it tell. To see this instinct given greatest range, turn to "Home Alone". Posing as a giggly exposé of Castle's obsession with interiors magazines, it is in reality a subtle disquisition on 9/11 and its bleak reverberations in the human psyche, locating intimations of mortality even in the Arcadia of Home Beautiful.

Despite the linguistic fireworks and catty comments, there's a sobriety at work here, and it's interesting to note that Sontag emerges from her debagging stripped of pretension but no less intellectually vital for it. As Castle says of her period of academic specialisation, the 18th century, one can sense beyond the "rococo lightness and drollery a deep moral seriousness humming away at the core". That same hum is certainly audible in these pages, though you might be hard pressed to catch it over your own delighted cackles.

Olivia Laing's To the River is published by Canongate