Leto: if you're trying to get a physical copy, you will need to do some from the Games Workshop site proper, NOT the Black Library site.
ALL physical books have gone through the main GW
Webstore since last year.
You will want to be refreshing the site on your browser around 10 AM, your region. In general, new items go up for pre-order at 10 AM. It's not unheard of for things to go up a few minutes before, though.
Make sure you're logged in and done whatever captcha is necessary before.
You can find all new items on their New and Preorders tab. However, I would probably recommend going to the Black Library tab instead, and set your filters (the "Refine" bar on mobile browsers, if you're on your phone) accordingly for Pre-Orders. This is to help pick pertinent books out from new model pre-orders.