Wandering through Rebecca Warren's Serpentine Gallery show is a pleasurable experience, until you find yourself among the dead. The title says it all: We Are Dead. Here are a bunch of grey clay lumps, paint daubed here and there into their gnarly surfaces, like the mould on funerary statues in overgrown cemeteries. I start to see faces and limbs, an ear, a mouth, a fist or a breast - but the human clay seems to be reverting to mud. This dumb assembly might be waiting for something, on their closely grouped plinths: I just hope it isn't me.  The feeling is of sculptures in the process of swallowing themselves. Nearby is another bigger lump, standing alone on a little wheeled platform. This isn't going anywhere. Something sticks out low down, like a little doggy's cropped tail or a come-hither finger. Most likely, it's a willy. There is also a long, springy bit of twig stuck in the clay, like a snowman's idea of an arm. This sculpture is called M.D and I have no idea why. Warren doesn't explain. Maybe it began as a statue of Marcel Duchamp, but the whole thing got called off. Lumps have their own satisfactions and pleasures, the possibilities one sees in them being the foremost. Like the hewn rock from which Michelangelo's Slave emerged, there may well be something in there, struggling to get out.  The more you encourage and coax it, the more malleable clay gets. Push it too hard, though, and it'll turn against you. Overwork it, and it can look like sucked toffee. Warren's clay figures exist somewhere between life and death. Her earlier figures, with their sprightly bodies, bulging breasts and gargantuan arses, those wrestler's thighs and thunking calves, have a kind of cartoonish, sexual monumentality; they mix Robert Crumb's fantasy Janis Joplin wonder-women and the hauteur of a Helmut Newton dominatrix - explicitly: the title of one piece is Helmut Crumb. This was the work that made Warren's name a decade ago. These sculptures were funny but limited, though everyone has to start somewhere.  What was really startling about Warren's early work was how unexpected it seemed - its expressive plasticity, those malleable, cloddish forms, even more than the sideswipes she was taking at her male antecedents. Ghosts still inhabit some of her sculptures: the sensual biomorphs of Jean Arp; the slanting, phallic form of Auguste Rodin's statue of Balzac; the minimal cube as imagined by Fred Flintstone. Lucio Fontana's ceramics, the early, faceted busts of Alberto Giacometti and Franz West's wonderfully laughable paint-smeared plaster sculptures are in there, too - though I might be imagining things.  In any case, Warren's art is more than a sum of sculptural quotations. Hers is not a tedious critique of male artists who have sculpted women's bodies - although in rendering Crumb's libidinous cartoons as sculpture, and morphing them with the sort of formal outrages committed by Willem de Kooning in his few sculptures, she turns them into something more than parody. These are stalking bunny-girls, their anatomical deformities a kind of empowerment. It always looked as if Crumb wanted his women to stomp all over him, anyway. Warren's approach mixes admiration for such artists with an equal unease about what they have done.  Some of her latest sculptures are made from welded steel. She mocked them up in wood and had them made by a craftsman. One is a sort of bench with a cleaver stuck in it. It looks ponderous and uniformly brown until you notice a squidgy white pom-pom resting at one end, as if this weightless little thing could balance all that heavy metal. Warren repeats this gag in another sculpture. Seen from the right angle, another vertical steel abstraction turns into a silhouette of a vertiginously slanted, fetishistic high-heeled boot, the sort Newton's models wore.  The largest rooms at the Serpentine are spacious and airy, but the scale of Warren's work makes for both intimacy and a kind of elegant formality. She uses crowding and emptiness to make her work start to sing. The first room is shared by her great big bulgy bronze cube and a smaller sculpture in a style I shall describe as infant-school rococo, which turns on a revolving dias. The large central gallery is a mix of old and new, of figures and dense, head-like globs. Given these differences, they all manage to talk to one another. It is as if one had walked into a studio where all sorts of things cohabit by chance. It can take a lot of work to fake such fortuitous meetings.  This sort of lively ensemble is what I think Warren is good at. Another gallery is lined with her vitrines, which remind me of Joseph Beuys's museological encounters between lumps of fat, rusty tin cans, razor blades and old batteries. Warren's slapped-together, wall-mounted boxes are more artful than they appear, with their grubby MDF frames, protruding nails, polystyrene oranges and moons, snaggly bits of sellotape, coloured curls of neon and dangling cables. The neon illuminates these interiors with a glow of red, a blush of orange, a blue chill; the longer you look, the more each seems to be a self-contained world, a sort of drama of found things and lost things, hair and lint and the stuff you find at the back of the drawer. Each little theatre has its cast of characters. Why is that chip of wood on his own? Was it something he said? Is a miniature Robert Morris going to walk in and rearrange those scraps of felt? Has a terrible crime been committed? It's a bit like visiting the reptile house at the zoo and peering into the gloom, looking for snakes. Get too close and that twig might poke you in the eye.  Part of Warren's skill lies in her sense of placement, the tension she creates between things. As well as losing herself in modelling forms, this kind of happenstance bricolage suits her. These changes of tempo and focus as one moves from room to room are very beguiling, as is the need one feels to stay alert - for the tiny pom-poms, a thing on the floor that looks like a rock erupting with tartan, a breast where a knee should be.  Warren has reached a point where the largest work can hinge on a detail, and a shocking incongruity can make all the difference. This is exciting for us as well as for her. She has only now begun to make dramatic relationships both within and between individual works, to be able to change the pace and tenor of a show with a sudden swerve. The possibilities seem wide open if she can grasp them.  Exhaustive show of collages and mail art in London's jaw-droppingly elegant new space.  • Raven Row, London E1: 020-7377 4300.  Drawings by the great sculptor, a century on from the Futurist Manifesto.  • Estorick Collection, London N1: 020-7704 9522.  The End of the Line  Eleven international artists in a touring show exploring the ongoing fascination with drawing.  • Mima, Middlesbrough: 01642 726720.
Adrian Searle is beguiled by the lumpy sculptures and dirty jokes of shortlisted Turner prize 2006 candidate, Rebecca Warren