Well, we don't actually know a book is released July. There should be one, as otherwise the release schedule is a little wonky (a book in March, then May, but not July). But stranger things have happened!
Anyway, if there is one in July, it should be announced shortly before or after Van Richten's is out. Announcements I believe always happen on Tuesdays (sometimes a little hint or something on the days prior) and an Amazon template as well prior.
So my guess is May 11, the week before Ravenloft's release.
As for what it is, I'm going to say a monster book in the style of Volo's and Mordenkainen's, with some additional subclasses included. Heavy dragon theme, but it may also include fey material as well.