David R. George III has two 24th century novels on the way, and on his Facebook page he has now revealed the first of those will be a DS9 novel, titled Sacraments of Fire. We can look forward to that in June, and George has previously suggested his next Trek book after that will be a follow-up.
Just ahead of that release, Peter David will be returning to Star Trek, with a new New Frontier novel in May. Peter David's website has that listed, with a link to an "untitled" listing on Amazon, which suggests the book will be released by Gallery Books, which means it will be in the larger trade-paperback format.
Many of the books for 2015 have been announced now, although not all the final titles and release dates are known yet. Hit the "prose" or "books" buttons on my 2015 schedule page for lists of all the known books so far.