Anchoring a Tall Bookcase to the Wall
A tall, narrow bookcase is stable empty, less stable loaded, and least stable when a drawer or low shelf is pulled or climbed on. Anchoring is not optional.
Why loading makes it worse
Books are heavy and most people load the upper shelves as enthusiastically as the lower ones. That raises the centre of gravity on a piece of furniture with a small footprint.
Fix into something solid
A bracket screwed into plasterboard alone is not a fixing. Find a stud, or use a fixing rated for the wall type. The strap or bracket supplied with the unit is only as good as what it goes into.
Level first
Anchor the unit upright and level. Pulling a leaning case flat against the wall with the strap puts constant tension on the fixing and racks the frame.
Load low
Even anchored, put the heaviest items on the bottom shelves. It makes the whole thing more stable and stops the shelves themselves bowing.