What is a good deed? And will it do you good if you do one? Mercifully, unless you are a Brownie (who of course must do a good deed every day), we don't need to find out, because Judith O'Reilly has done it for us.

O'Reilly, who sprang to bestseller fame by writing Wife in the North, a book about moving up to Northumberland and then not liking it very much, had the bright idea of doing a good deed every day. For a year. And writing a book about it. At the back, there is a helpful list of her good deeds; they include looking for a lost dog, giving some money to charity, picking up litter and going the extra mile for her parents.

Now, while many of us do the above, and more, on a pretty regular basis, and notwithstanding the fact that O'Reilly seems to have had a book deal in hand before embarking on the deeds, which slightly takes the sheen off her achievement, this is a funny, uplifting and admirable book.

What it is, more than anything else, is a profile of a small but energetic community where people help one another out, share the load of an autistic child, a blind mother, a teenager who needs guidance. I'd quite like to visit, because the life that goes on up there, on a windy perch somewhere near Lindisfarne, sounds fun, if rather chilly. I'd also like to meet O'Reilly herself, whose agonies about how to square career, motherhood and speaking in a secondary school assembly are something many readers (myself included) will find themselves grimly laughing in agreement over.

"Our children are our gods, and they are demanding gods," she writes, "our home is our church. Who has time to volunteer when you are a member of that religion?" Quite. But she's got no choice. She's told her husband and the kids that she's going to do a good deed every day. By February, the village has cottoned on. People are ringing her up and asking her to do their plumbing for them.

Why this book succeeds is because O'Reilly is not some patronising do-gooder whose life is terrifyingly perfect, but a real, warm woman whose chaotic world as a not entirely successful freelance hack bounces from unpaid phone bills to broken-down cars via three squawking children and a husband who insists on taking long working trips to London.

She is very funny about the volunteering sector, which she finds rather less charitable than she thought she might (the Salvation Army and the lifeboats are both rather resistant to her enthusiastic pleading), and it is the invention of her own charity, the Jam Jar Army, aimed at raising money for causes in Northumberland, that gives the book its necessary backbone and narrative thread. There are several rather sweet good causes who turn up at regular intervals and some deeply tiresome teenagers whom O'Reilly insists on helping, to the extent of writing CVs for them at midnight, and who are most gratifyingly ungrateful for her efforts.

Where O'Reilly really comes to the fore, however, is not in her self-deprecatory comic writing (of which there is a pleasingly large dollop), but in quite the opposite. Every so often she returns to the unalterable fact that her first child, a son, was stillborn at term. She writes about bereavement in such a clear-eyed, unafraid yet somehow positive manner, it makes you hope she spends her next year volunteering to counsel the similarly stricken.

Does the year change her? Overlooking the fact she celebrates the end of it by knocking back the booze and announcing: "Tomorrow I'm not doing another bloody bollixing good deed all bleeding day," it does. It has given her purpose, and it has made her happier. And while most book deals will give you the first, not all will deliver the second.