How important is it to write a series rather than a stand-alone title?

I'm delighted to tell you that the third book in my bestselling series about the Duke of Silchester, "The Duke's Alliance", and his five siblings is now available on preorder. I'm writing two a year and the fourth one will be published the end of the summer. I can't wait to get onto the duke's story – although he does feature in every book.
This is the second Regency series I've done – the first was the three book 'At Pemberley' series of Jane Austen variations. Although I enjoyed writing these I was glad to write the final one as using somebody else's characters somewhat stifles the creative impulses.
Currently I'm writing the third book in the Victorian saga, "The Nightingale Chronicles", and there will be a fourth next year.
 I've also written the first book in a World War II series, "Ellen's War", Blue Skies & Tiger Moths, which is going to follow the life of a woman ferry pilot in the ATA. This will be launched in April. I'm very excited about this particular book as there aren't many fictional stories about these brave women.
I've written a stand-alone Regency which has yet to be published, and will write a Christmas themed Regency – but these will be the only stand-alone titles I do this year.
Series seem to do so much better than individual titles at the moment – I certainly enjoy reading about the same character in Bernard Cornwell and Christian Cameron's  brilliant series – so I suppose I'm not the only one.
This book is available as a pre order at the moment and will be released on 23rd  February.
Here is the blurb:

£1.99 /$2.99
Release day 23rd February

An Unconventional Bride is the third in The Duke's Alliance series. 
Mrs Mary Williams, a colonel's widow, arrives at Silchester Court with Miss Elizabeth Freemantle, who has been brought up as her sister. Beth is the Duke of Silchester's cousin and he is her guardian. 
Lord Aubrey, the duke's youngest brother, finds himself designated to oversee the London debut of both Lady Giselle, his sister, and his lively cousin, Beth, as the duke is called away to his estates in the North.
Although Mary is only four years older than Aubrey she is more worldly and well-travelled. Mary is not thinking of marrying a second time, she values her independence too much, and certainly not to a young gentleman like Lord Aubrey. 
Only when her reputation is lost, and marriage to Aubrey is impossible, does she understand that her feelings have changed.
Is it too late for them to find happiness together? Will the duke allow her to be part of his prestigious family?

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Fenela J Miller