I still use books (and so does my team), because I'm not always 100% sure that what's in the encoders/software is precisely what the AMA has published and that's the tool that the AAPC expects you to use for the exam.
That having been said, notes in your book are fine, but the point of the exam is to ensure that you know how to find the correct answer. The best way to ensure that, without marking up your books with a lot of notes that may not help later (and waste your time in the process), is to ensure that you know the book's layouts. Knowing where to find information to validate the correct answers is the key to passing this exam. Did you know that Optum's version of HCPCS II has a glossary? That the AMA Professional version has a chart related to needle EMGs and how many studies to report for each body area? You do not need to memorize the codes. You need to know the rules for code assignment and where that information is written.
And think about how to find the 'right' answer. For example, one question on my COC exam asked me to identify where the axilla was located. I drew a blank, even though I absolutely knew this. I went to the back of the CPT book, looked it up, and went to one of the related CPT codes in the book, which identified it as not being any of the other three answers.