Dumbledore’s Army: The Dark Side of the Demob is a tell-all book by Rita Skeeter about Dumbledore’s Army. In this book, published on July 31st 2014, Rita described the imperfections of the Dumbledore’s Army members. She promoted it during the 2014 Quidditch World Cup in her article “Dumbledore’s Army Reunites at the Quidditch World Cup Final” (Pm).
"Demob" is the abbreviation for "demobilization," which means "ceasing military operations or withdrawing troops".
The word demob may refer to Dumbledore's army never being part of a military interference, so it would mean a group of ineffective fighters. Also, the word seems to refer to "the mob", which is "a large crowd of people, especially one that is disorderly and intent on causing trouble or violence", according to the Oxford dictionary.
Also, note the date the book is supposedly published: on Harry's (and Jo's) birthday!
From the Web
Screenshots of the Daily Prophet's coverage of the 2014 Quidditch World Cup: http://imgur.com/a/AXutv