Benj is reading his way through the Renaissance and Reformation with books from Ambleside Online Year 8 and most of his Commonplace entries have been inspired by those books. He chooses his own quotes but when he first started keeping a Commonplace book and happened to comment on something that impressed him while he was reading, I would suggest he record the passage in his book. I don't do that anymore because the habit is in place. I just like reading what he has written and to see what books have kindled his interest enough that he would record something from them.
Recently, he has quoted mostly from Churchill's The New World and Whatever Happened to Justice by Richard Maybury.
It is very helpful to read with a commonplace book or reading-diary, in which to put down any striking thought in your author, or your own impression of the work, or of any part of it; but not summaries of facts. Such a diary, carefully kept through life, should be exceedingly interesting as containing the intellectual history of the writer; besides, we never forget the book that we have made extracts from, and of which we have taken the trouble to write a short review.
Formation of Character by Charlotte Mason
Jonathon Swift, the author of the book Gulliver's Travels amongst others, wrote a letter to a young poet in 1720 with this advice:
A commonplace book is what a provident poet cannot subsist without, for this proverbial reason, that "great wits have short memories:" and whereas, on the other hand, poets, being liars by profession, ought to have good memories; to reconcile these, a book of this sort, is in the nature of a supplemental memory, or a record of what occurs remarkable in every day's reading or conversation. There you enter not only your own original thoughts, (which, a hundred to one, are few and insignificant) but such of other men as you think fit to make your own, by entering them there. For, take this for a rule, when an author is in your books, you have the same demand upon him for his wit, as a merchant has for your money, when you are in his.
Benj is using a combination of Apologia Physical Science and Understanding Physics by Isaac Asimov. He loves the detail in Asimov's book and uses Apologia as a general overview and for experiment ideas and this has been working quite well. He's almost finished the Apologia book but will continue with Asimov and start doing Biology, which I'm in the process of planning.