Don't get me wrong doing the SAT questions on here will be extremely helpful in preparing for your ACT, but just make sure you know the format of the ACT.

The official book: https://www.target.com/p/the-official-act-prep-guide-2020-2021-book-5-practice-tests-bonus-online-content-paperback/-/A-79489877

Study guide: https://www.amazon.com/Premium-Study-Guide-Practice-Barrons/dp/1506258255/ref=sr_1_1?dchild=1&keywords=barron%27s+act&qid=1593855096&sr=8-1

Flashcards (essential): https://www.amazon.com/ACT-Flashcards-Barrons-Test-Prep/dp/1438079036/ref=sr_1_8?dchild=1&keywords=barron%27s+act&qid=1593855110&sr=8-8

Barron's ACT: https://www.amazon.com/Barrons-ACT-2nd-Book-Only/dp/1438007965/ref=sr_1_10?dchild=1&keywords=barron%27s+act&qid=1593855110&sr=8-10

Grammar Workbook: https://www.amazon.com/SAT-ACT-Grammar-Workbook-Barrons/dp/1438008732/ref=sr_1_15?dchild=1&keywords=barron%27s+act&qid=1593855110&sr=8-15

...and any other book you can find Barron's related

Also search up books by the Princeton Review; they're good.

Erica L Meltzer has some incredible workbooks:

English: https://www.amazon.co.uk/Complete-Guide-ACT-English-2nd/dp/1530072808

Reading: https://www.amazon.co.uk/Complete-Guide-ACT-Reading-2nd/dp/0997517824

I would also search up any books you can find written by her (and also consider buying some SAT books she has written; they're really clear and she makes her own practice problems so you're not wasting official SAT past questions).

Hope the helped and good luck for whenever you sit your ACT!