As I started a thread about this subject a while ago, so that other budding authors could follow my progress, I have had a few months of first hand experience.
But this is only my own advice as a real novice, so I can only tell it as I see it through my eyes.
Firstly, there are too many cookery books out there, but people still write them, there are too many florists, but someone will open a new one and be successful. Another example is this forum, there are too many Anglo/French forums, but I would say this new one is doing particularly well. This all comes down to how well you do the job and promote your business or novel. Yes it can be a hill to climb, but just because things have been done before, doesn't mean they cannot be done again.
I have written a little fictional novel based in France, I wanted a different angle, so I chose 'white van man' moving to France, so it is miles apart from 'A Year in Provence' and books of that ilk. But I was sure there was a market for it, albeit a far smaller one. I wanted to write a book, so this was just going to be a trial to see if I had it in me.
To get it taken up by a publisher, I knew would be virtually impossible but I still tried, then I stumble along Amazon publishing. I had no fancy programme, just WORD on my pc, this was more than enough to upload the book on to Amazon for Kindle and now Create Space for a paperback version.
The results so far:
Vantastic France was launched on Kindle early January 2012, I have had nearly 800 downloads, some were free as promotion days, but I am now selling about 5 to 8 downloads a day already.
The paperback version is just out and in the UK due to competition from suppliers that can buy it from Amazon, the price is now £5.99 in the UK.
I am quite pleased with my results, it also shows that books about France are far from over done. I still buy them every week myself, especially as some are just £1.99 on Kindle.
Yes, some will not a appeal to everyone and one that looked well rated turned out to be just a copy of someone's blog, so not much of a story line there, and you could say a waste of money, although I have noticed that the latest reviews were not very good.
So I would use your diary to create the story, you have lots of material to work with which is a great start. You could also choose to write it as fiction so you could then add a little spice to it, or non fiction which I understand is slightly more popular.
As I have said, I am a total novice and I do not regret one second of the hours it took me to write my 75,000 word novel.
I have a blog stevebichard.com which gives some advice on how I managed to get published, with a few tips and links about setting up your book on Amazon.
I could see old established authors looking down their noses at Amazon self-published authors, as anyone can now put a novel on the market. However, that is progress and the strongest will still survive, it's just that a few of us are now having a little slice of their pie.
As the old adage say, 'just go for it.'