The Warmaster by Dan Abnett, the fifteenth novel in the highly popular Gaunt's Ghosts series, has been finally scheduled by Black Library for release in December this year.
The book is the third of four novels in the Victory sub-arc (to be followed by a novel provisionally entitled Anarch), itself apparently the penultimate series in the much larger Gaunt's Ghosts series, with possibly two novels in the final series (subtitled Archon) to wrap up the saga. However, these plans may have changed.
The Warmaster was originally scheduled for release in 2013. The reasons for the lengthy delays are unclear: Abnett was diagnosed with epilepsy in 2009 which delayed his work on the Horus Heresy series as he adjusted to medical treatment, but apparently the problems caused by this are years in the past. Abnett has also been working for Marvel Comics, his work in demand since he created the modern iteration of the Guardians of the Galaxy series (which the two movies are based on). However, Abnett has always produced a prolific amount of comics work alongside his novels without issue in the past.
More likely is the fact that Black Library and its parent company, Games Workshop, have been going through numerous convulsions and changes in the last few years. They have nuked their classic Warhammer fantasy setting, lost several high-profile authors and, bizarrely, deleted the omnibus editions of their novels and reprinted the individual books for a higher cost than the omnibuses, which has gone down like a lead balloon with fans and has put off potential new readers. Games Workshop's release schedule has slowed to a crawl recently with very few novels put out, the Horus Heresy series still nowhere near a conclusion after eleven years and forty-one books and the company seemingly focused on finding ways of selling existing material rather than producing new work.
Anyway, the good news is that The Warmaster is coming out and hopefully we will see the remaining books in the Gaunt's Ghosts series quite quickly afterwards. Abnett is also working on Penitent, the second novel in the Bequin trilogy (ending the storylines begun in the excellent Eisenhorn and Ravenor trilogies), which is likewise eagerly awaited by fans.
There will also be a companion novel to The Warmaster. Matthew Farrer has written a stand-alone Space Marines Battles novel called Urdesh which will take place simultaneously with The Warmaster, but it will not be necessary to read both to enjoy either novel.
Meanwhile, there is no word on Paul Kearney's Warhammer 40,000 novel Umbra Sumus. The book was pulled from release in 2015 when its series title, Dark Hunters, was ruled as infringing the copyright of Sherrilyn Kenyon's urban fantasy series Dark-Hunters. GW has yet to announce if the book is going to be renamed and reissued. Kearney has recently released a new Warhammer 40,000 novel in the Space Marine Battles line, Calgar's Siege, however.