I own dogs but have also suffered from dogs barking next door. Dogs generally cause a problem from excessive barking when they are left alone for long periods and left outside. The one time that our dogs caused an annoyance by barking was when my daughter let them out into our garden and then went off to her room and forgot to let them back in - she was listening to music and didn't notice them barking but of course the neighbor did! The dogs were just asking to come back in. An apology from me and taking care not to let it happen again sorted out neighborly relations. We suffered from dogs barking when living in a previous house where the neighbor left a couple of dogs outside in a concrete run all day - the dogs barked because they were bored, we were only renting so it wasn't a long term problem.
A good way for owners to train dogs is a remote control collar that squirts lemon juice, it does the dogs no harm but they hate it and it draws them up short if they are up to any naughtiness.
I think the best approach as others have suggested is speak to the dog owner, they may not be aware the extent that their dog barks as they are probably out when it happens. If the owner isn't cooperative then the RSPCA probably can give advice, they may consider that a dog that is left alone for long periods is not being properly looked after and take action.
To lighten the mood there is an old joke (I'm not making light of the real problem of barking dogs but I do like this joke) :
A couple suffered from a dog barking next door. One night the man said to his wife "I've had enough, I'm going to sort this out!", he disappeared and there was the sound of a skuffle and then the noise of the barking got a lot louder. The man came back into the bedroom with his pajamas torn and mud and dirt on him but looking very pleased with himself. "What did you do?" his wife asked. "I've put the dog in our garden, now see how the neighbor likes having a dog barking next door!"