Shaikh Abdul-Muhsin Al-Abbaad was asked the following question:
Oh my honorable teacher. Some People of Knowledge advise to record the benefits by writing it in a notebook because it will be well-organized and clear. Some advise to write in the book so that it can be studied more when reviewing. So what is your advice for us?
By Allah, each is good. However, if a person has a notebook to record in or if the margins of the book are spacious that it suffices, then there is no harm in this because he can distinguish what is printed from his handwriting. However, if a person has a notebook that he records benefits, then no doubt this is good. However, if he writes in the book and (it is well know that) every book has marginal notes, then there is no harm.
[Sharh Sunan Tirmidhi no. 290]
Faisal Ibn Abdul Qaadir Ibn Hassan