hi, i personally take biology and i think having a cie biology text book is a must. if you want a reference book with greater depth, then try biological sciences. it explains concepts in a more detailed manner. however, bear in mind that a lot of those details will not be tested in your actual exams. Nonetheless, if you want to improve your understanding of the topic, going through the details might help you remember the process and topic better hence getting better results in exams.
Also, do lots of past years to familiarise yourself with the questions and mark scheme!