Scrum.org, the body that administers the PSM I exam, offers its own free Scrum Guide, free Open Assessments, and paid Applying Professional Scrum training. Because it's the source of the exam itself, its materials are as authoritative as it gets — there's no question of alignment with what's actually tested.
What Scrum.org's official resources don't do is track your individual readiness over time. The free Open Assessments give you a one-time score, but there's no adaptive plan built from your results, and the paid live training runs on Scrum.org's Professional Scrum Trainer network on their schedule, not yours.
Edureify isn't a replacement for reading the Scrum Guide — it's built on top of the same source material, with a Diagnostic that tells you specifically where your understanding is thin and a roadmap that keeps you there until your Readiness Score says otherwise, which the Open Assessments alone can't provide.