The official ACT guide is enough for most students if you actually take the tests timed, score the enhanced core, and review every miss. Buy a second book only for a named weak skill the official explanations do not teach.
When the guide is enough
You can finish sections, your error log has a few repeating types, and you will sit two official-style cores before the date. Free extras: free practice tests. ACT's own platform: official online prep.
When to add a book
English grammar is a mess, Math content has holes, or you want more explanations than ACT prints. Stack, not a swap: best prep books.
The bottom line
Official questions first. A third-party book is a tutor for one skill, not a second Composite.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.