I am not much one for making commercial pitches, but I am quite pleased to have recently found a series of small, flexible notebooks by Field Notes Brand. They are pocket-sized and paper-covered, so they fit easily in back pocket or coat and are invaluable on jobsites. With a significant number of projects currently under construction in Boulder and Chicago, they have been my constant companion lately and the repository of many on-site generated changes and details. http://fieldnotesbrand.com/
They are not quite large enough to replace the classic 5x8 landscape-bound black sketchbooks that I have been using for 20 years, but they are much more likely to make it to and from the jobsites.
On top of a bookcase in my office is a collection of about 30 or so sketchbooks, large and small, that I have employed since undergrad. These include the aforementioned 5x8s (the landscape-bound version can not be purchased here in Boulder and has to be ordered), as well as Moleskins, innumerable spiral notebooks and some very nice Japanese sketchbooks. Chock full of drawings, sketches, ideas and the occasional grocery list, it is a growing company with the Field Notes soon to be added.
It is hard to imagine the number of really bad ideas that are in these sketchbooks that thankfully never saw the light of day.