Every Breath by Ellie Marney (Tundra hardcover, 14 October 2014).
Rachel Watts hates living in Melbourne, but moving to the city was a financial necessity for her family after they lost their farm. But at least she made a good friend fairly quickly. James Mycroft, also seventeen, lives two doors down and goes to the same high school.
Mycroft (they are on a strictly last-name basis, Rachel tells us -- the story is told in the first person from her point of view) lives with his Aunt Angela, with whom he has a rocky relationship. We learn the details of his parents' tragic death later in the story.
Late one evening, when Mycroft is showing Watts around the city, they stumble upon the body of one of his pals, a man known as "Homeless Dave". While they are waiting for the police, the teens take detailed photos of the crime scene, and after they spend many hours being interviewed at the police station, Mycroft concludes that Dave's case won't get much attention because of being homeless.
He resolves to solve the case on his own, and Watts, reluctantly at first, finds herself helping him. Though both of them are extremely intelligent, they inevitably get into hot water at home, at school and just in general.
One of the noteworthy things about this story is that both teens joke about their names. At one point, Rachel reminds her friend that Mycroft was Sherlock's smarter brother.
The relationship between Rachel and James does develop into more than a friendship, and it will be interesting to see how it evolves in the next two books in the series, Every Word and Every Move. Both of these have already been released in Australia, but there's no indication of when they'll be published in North America.
FTC Full Disclosure: I borrowed this book from my local library.