I’m assuming you’re talking about the CFA Institute curriculum, since the only material from CFA Institute that is called ‘Handbook’ is the The Standards of Practice Handbook, which is useful only for Ethics.
There isn’t really a ‘best’ study method, as it differs from candidate to candidate. I can tell you that most candidates use third-party provider materials – some to support their learning from the official CFA Institute curriculum, but a lot of them use third-party materials exclusively.
A lot of candidates do find third-party materials beneficial, by making studying more exam-focused and efficient.
Many candidates find it helpful to use third-party prep providers who offer study materials specifically tailored for the CFA exams. Some popular providers include Kaplan Schweser, Wiley, and Bloomberg, among others. These providers usually offer study guides, practice questions and online resources to help candidates prepare effectively. Haunt The House