Having a copy (or two) of the book makes it easy for underlining your favorite passages or otherwise keeping small notes assigned to various sections.
If you prefer, instead (or in addition) to reference an online copy of the Course, then take a look at this terrific resource from Miracle Center. Here you can look up any passage from the text, workbook, or manual – including complex searches using single words or entire phrases. Very handy indeed!
For the most comprehensive Q&A related to A Course in Miracles, check out the Foundation’s Question and Answer Service.