My rule of thumb is that you can send out a news release offering books for review as long as:
1. you have books for the media to review and can ship them within a week or so of getting the request
2. the book is available for purchase on Amazon and your web site at a minimum (and maybe other places as well).
Timing is roughly based on when you want the publicity to hit taking into account the normal lead time the media needs to do what they do even if they decide to do a feature story, an interview or a book review the day your book arrives in their hands. Your web site and Amazon need to be in place and operational so you derive the financial benefit of your promotion.
Now from experience (and I do send out lots of news releases for authors and publishing companies), 95 percent of the media response to a news release transmitted by email occurs within 24 to 48 hours of delivery. Then you have to stuff a package (with a book and a copy of your news release at a minimum), send it by street mail (I recommend First Class or Priority and never media mail), then wait for delivery (five to seven days), then wait for the media to review the book or take action (review time on the book and work time to take action on your proposal for media coverage).
The minimum lead time for media is usually two to three publishing cycles. So for online media you can estimate that even once they get the book and if they read it right away, it’s one to two weeks. For daily newspapers it is two to three weeks. Same for radio. TV varies on the urgency and perceived interest but the normal lead time is three to four weeks. For magazines, the lead time if four to six months.
I recommend you transmit your news release on timed for Tuesday, Wednesday or Thursday delivery. The media tend to operate on a five day work week. Saturday and Sunday they are gone. Friday tends to be a bad day because they are wrapping things up and trying to get their desk cleared before the weekend. Monday is a bad day because they are catching up from Friday and also have staff meeting and priority assignment from on high. By Tuesday, you can get the maximum attention to read and respond to your proposal and it gives the media the rest of the week to identify, allocate and deploy the resources needed to give you the type of publicity you seek.
You may think that sending an eBook will expedite reviews. Don’t make that assumption. Media seek to validate when they get a review copy. They want to make sure they recommend something good that has real value. The ebook or pdf file is too readily deleted, discarded or ignored for you to trust a media to do a quality review. The best way I know to maximize your success with media is to make a quality presentation and give them what they need to do the best job you want done. Send them a quality book, a quality pitch, photographs of the cover and the author, plus feature story quality content and value.