Problem: Nook memory is full. The steps below are specific to ADE 4.0.2 and Windows Vista.
Solution: Reduce number of items in Nook memory by doing the following: (1) (a) Load ADE, (b) Plug in USB cable into PC, (c) Wait for "Nook" to appear on screen at left just above "Bookshelves", (d) Left click on "Nook" - contents of Nook's bookshelves will be listed, (e) Decide which titles you wish to delete - select (left click) appropriate title(s) - for multiple title selection hold down "Ctrl" key as you select, (f) Keeping cursor on any selected item, right click for dropdown menu and choose (ie., left click) "Delete", (g) Continue process until you have reduced the Nook's memory contents to an appropriate amount. Note: Items will only be deleted from the Nook, not from the ADE Bookshelves.
(2) The procedure outlined in (1) only will remove the main text files - the contents of the book. Your reading device (in my case, a Nook) may have other types of files in its memory. These may be removed by doing the following: (a) Plug the USB cable into the PC, (b) When the "AutoPlay" menu appears, select "Open folder to view files", (c) Double left click "Digital Editions" in the window that appears as a result of (b). (d) Select the file or folder content you wish to delete, right click and select "delete". Note: These additional files do not usually take up much room in memory so procedure (2) may not be necessary.