Yeah, I wondered why it wasn't actually on the Osprey site.
Then I noticed they only publicise books that are due for publication in the next 3 months only.
I suspect it's still pretty much early days for Outremer, and it wouldn't surprise me if April 2018 is just a speculative date.
I have found Osprey's 'hardback' rules (e.g. Frostgrave - which I've played and enjoyed, Bolt Action - never played but heard enough to know it won't be for me, Horizon Wars - just didn't know what it wanted to be, Force On Force - not my sort of thing, Fog - that game is the work of Satan and everything I dislike in miniatures' games, plus I think there was an AWI one and an ACW one?) have left me cold.
But those 'blue' rulebooks are the right physical size, self-contained, and at the right price that I am willing to take a risk on them. I think they've been a real boon to the hobby.
I'm also heavily in favour of period-specific rules, but not too narrow and specific. The Crusades could also be covered quite effectively by Lion Rampant, for instance. Sometimes, when the rules writer is trying to make the rules specific, they let too much get in the way. I hope that Outremer doesn't fall into that trap. I shall happily look forward to it though and in the mean time paint the odd Saracen or two.