In regards to my own writing, three out of four of my completed manuscripts have followed whatever was trending at the time. This was not intentional. It is not that I study the market religiously and write what is hot at the time; it is just that I like to read a wide variety of genres and whatever is on the bestseller lists. And like night follows day, what I write tends to reflect what I read.
Case(s) in point: In the 1990’s, after the advent of the female sleuth thanks to Sue Grafton, Janet Evanovich, Patricia Cornwall and others, I wrote my first manuscript which centered around a female sleuth. Helen Fielding’s successful Bridget Jones’ Diary not only spawned a movie but a whole new genre: chick lit. That was my second manuscript, after I’d spent years reading just about everything in the genre. Then Stephenie Meyer had a mega hit with the Twilight series and I had to read them to see what the fuss was about. An unexpected bonus was that it introduced me to YA (young adult) and despite my age, I found that I really enjoyed it. Meg Cabot, anyone?
In saying all of that, I think that it is important to be aware of what is trending and what is not and remember that the nature of the beast is cyclical. Chick lit is all but dead they say and people now suffer from vampire fatigue. But there will always be buyers of those books and it may trend again. If you were to get an offer on your book today, the reality is that it wouldn’t be published for another one to two years, so whatever is trending right now may not be trending then.
I think it is important to keep your finger on the pulse of the market, make a mental note of it but at the end of the day, write what you love.