https://www.warhammer-community.com...ingers-the-next-chapter-of-the-age-of-sigmar/ There is a roadmap. For summer seraphon battletome, one narritve tome and new ghb are planed. We already got our tome. So probably 1 month max 2 month till new ghb
Honestly all the rules are online, you can find them. If you haven't been playing competitively just stick to what you have been playing. If you haven't been playing, play the core rules until you and your friends are comfortable with the base rules. The GHB adds new rules that can be more complicated if you are new.