As details of JK Rowling's first novel aimed at adults have begun to leak, a clearer picture has emerged of the most anticipated book of the year and the millionaire author who fiercely guards her privacy.

Rowling, who shot to fame and riches as the author of the Harry Potter series, will on Thursday publish her new book The Casual Vacancy – the most anticipated release of the year.

In recent days, she has talked about her troubled relationship with her father, her unhappy teenage years, a failed marriage and a battle against depression. Details of the book also reflect her fears of increasing inequality in Britain, and support for the welfare state that she needed when writing the first Harry Potter book. The drip-feed of hints has even led residents of her childhood village, Tutshill in Gloucestershire, to deny accusations that "snobby" attitudes in the community provided inspiration for her pretentiously middle-class characters.

The buzz around The Casual Vacancy has continued to grow, but that was not always the title. Rowling told The New Yorker magazine: "In my head, the working title for a long time was Responsible, because for me this is a book about responsibility."

Not just "how responsible we are for our own personal happiness and where we are in life, but for the macro sense also of course: how responsible we are for the poor, the disadvantaged, other people's misery", she said.

The eventual name came from a handbook for local administrators. A casual vacancy refers to a seat becoming vacant following death or scandal. Rowling said: "Immediately I knew that was the title." While the Harry Potter books largely left politics alone, Ms Rowling is known for her left-leaning views. In 2008, she donated £1m to the Labour Party saying the "poor and vulnerable will fare better" under the then Labour Government, rather than a Conservative Party led by David Cameron.

The author pays full taxes as a UK resident. She once said: "I am indebted to the welfare state … When my life hit rock bottom, that safety net, threadbare though it had become under John Major, was there to break the fall."

Work on The Casual Vacancy started before the general election but Rowling said the benefits situation had become worse since the Coalition Government came into power. The book will tackle distinctly adult themes in what she describes as a "comic tragedy" and explores the closed society of a small town. It focuses on the impact of a local election, calling it the "smallest possible building block of democracy – the tiny atom on which everything rests".

The Casual Vacancy has received the largest number of pre-order sales this year, according to Waterstones. It comes five years after the release of the final instalment of Harry Potter. The seven-book series sold more than 450 million copies worldwide.

The contents of The Casual Vacancy have been closely guarded until this week when a few reviewers have been let into the publisher's offices to read the book subject to onerous non-disclosure agreements. In promotional interviews, Rowling has talked further about the difficult relationship with her father, noting that one character in the novel mirrors her teenage wish to move away to London.