An inventor's log book is used to record the progress of your inventing. You should start using one the moment you think of an idea for an invention. However, your log book must be of a certain type.
You can buy a specially printed inventor's log book. You can also buy a generic bound notebook. The most important thing is that the pages of the notebook cannot be added or subtracted without that being evident.
Before Buying Specially Printed Log Books
Look for sequentially pre-printed numbered pages, fade-away backgrounds, spaces for you and a witness to sign & date, and instructions on how to use the journal. Look for pages with blue-lined grids for easy drawing. Some log books have special copy features; copy drawings on a light copier setting and the grid pattern fades away for preparing patent application drawings, or copy the drawings on a dark setting and the words, "Do Not Reproduce" appears for confidence usage.
Generic Bound Notebooks
Never buy a loose leaf notebook. Never buy 3-ring binders to use as a log book. Never buy a legal pad or any glued together notebook. Buy a notebook with pages as secure as possible - a bound or sewn notebook. Mead brand composition books are perfect. Buy only notebooks with white pages - the lines can be colored blue or black.
Generic Ledger Books
These common and inexpensive ledger books can also be used as a log book. The same considerations given to bound notebooks will apply - bound books only. Remember you have to buy a separate log book for each different idea, so inexpensive is sometimes the way to go.