Just got an email regarding my pre-order, this has been pushed back until July. Bummer! I heard the book got pushed back until the end of April?
The original release was scheduled for the end of April (right around Alien Day), but as of today it is now slated July. There doesn't seem to be any word from Rinzler on Twitter as to why it was delayed.
Looks like the Alien Blueprints book, which was also to be released in April as well, was moved to September, too. Don't have that one on pre-order though, so I didn't get an email.