Where can I find information on wall bracing requirements at corners with openings?

The requirement for panels or let-in bracing in the corners of conventionally-constructed walls is, in general a non-engineered detail that provides lateral resistance of the wall assembly and structure when subjected to lateral loads such as occur during wind or seismic events. Overturning restraint, in this case is provided by the dead load acting on the wall and/or anchorage provided by perpendicular walls. However, when openings occur near the corners, the aspect ratio (height/length) of the wall located at the corner will vary with the type of construction (braced wall panel vs. shear wall) used as well as the size of the opening.. 

Basic Requirements
Section R602.10.4 of the International Residential Code (IRC) provides basic wall bracing requirements and specifies the minimum length of braced wall panels for different types of exterior sheathing - in general this is 4 feet. Therefore, where an opening exists near a corner, the minimum length of braced wall panel required (at the corner and elsewhere) is 4 feet for an 8 foot high wall.

Using Continuous Structural Panel Sheathing
Using wood structural panel sheathing (OSB or Plywood) on all sheathable areas of all exterior walls and interior braced wall lines as well as having corners constructed in accordance with IRC Figure R602.10.5, can reduce the minimum length of braced panel required as per Table R602.10.5 (e.g. can reduce the min. 4 foot length to a 2 foot length of braced wall panel for an 8 foot high wall, or to a 2-1/2 foot length for a 10 foot high wall - provided the maximum opening height next to this braced wall panel is 65% of the wall height.

R602.10.6 provides further options where one can essentially use 2 foot 8 inch panels (min.) for walls up to 10 feet in height with no restriction on max. opening height - but this requires the use of tie-down devices, and other additional fastening and sheathing requirements.

Note that recent code change proposals for the upcoming 2006 IRC are looking to permit walls with 6:1 aspect ratios used with continuous structural panel sheathing.