Why I keep a commonplace book
Copying out other people's sentences by hand is the slowest and best way I know to read.
A commonplace book is a notebook of quotations, copied out by hand and loosely arranged by theme. Locke had a system for indexing his; Virginia Woolf kept hers in fat bound volumes. Mine is a cheap notebook and a text file.
The value is not in the collection but in the copying. You cannot copy a sentence by hand without reading it at the speed it was written. Halfway through, you notice the comma you would have skimmed past.
The index
Every few months I add an index to the back pages: one word per theme, followed by page numbers. It is the only part of the notebook I ever read in order.