Through the wormhole with flippers and fins: Jamila Gavin, The Wormholers (London: Egmont Books, 2003)
And by this time I can't help noting how many of the sf books come from small presses even where--as with Jamila Gavin--the author's fantasy is with a mainstream house.
The Wormholers is interesting for what it doesn't do. It doesn't explain much. It is dull exactly in those moments when it takes time out to give a little lecture because those lectures are not about the science of wormholes (that's done pretty well) but about morals--caring for your stepmother/handicapped neighbour/the earth.
Chad notices the baking powder trying to hide itself on the shelf one day. He ignores it. Then the next day the condiments move in front of his eyes, the kitchen looks huge and he passes out. A chap comes by and asks him about things disappearing. He denies it. Then two pieces of lego fall down a sudden chasm in the floorboards. Knowing that three experiments makes proof, he calls his disliked step-sister over. Natalie disappears and Chad panics. He calls the strange chap back by inserting the calling card into the computer as if it were a disk. Strange chap reappears and offers to take him into the wormhole to look for Natalie. Chad agrees but--and here is one interesting difference--the wormhole closes too late to take him, but leaves him with the chap's wormhole dousing equipment.
With Natalie gone Chad finds himself filling in for her in entertaining their neighbour Sophie who has cerebral palsy. But today Sophie has been given a computer which can be operated by a tilt of her head, and it turns out that Sophie is a genius.
From here there are three plot lines. Chad's search for his step-sister which takes him through the interstercise of space and time to a meeting with his half-brother in Australia. Natalie's experience living in a backwards dimension in which her father is once again alive. And Sophie's experience in another body--one we eventually realise is a whale--as part of an overmind which tries to protect planets from humans.
The bit I liked least was that Sophie opts for her crippled body over freedom (home is apparently best) and there is some mawkishness for Chad with a teddy bear. But surrounding all that is some very interesting discussion of the science and theory of wormholes and time and space, and a fair bit of philosophy too, all disguised in a rather interesting portal fantasy. There is no consequence at all at the end (except some personal reconciliation) but interestingly, I felt satisfied.