The ornamental design for a notebook, as shown and described.
FIG. 1 is a perspective view of a notebook showing my new design, with the notebook string wrapping around the top, front, and bottom of the notebook to keep the notebook closed.
FIG. 2 is a front elevation view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a left side elevation view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a top plan view thereof.
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view thereof, with the notebook string unwrapped to show the inside of the open notebook.
The broken lines showing a pen form no part of the claimed design.