Boston--HookMedia Inc., one of the top b-to-b Internet advertising media planning and buying firms, said Tuesday it has opened an office in Atlanta. The move is the first in a series of expansions, aimed at giving HookMedia a nationwide presence. The company specializes in planning, placing and measuring direct marketing campaigns run on the Internet. Its b-to-b client list includes Allaire Corp., EMC Corp., PricewaterhouseCoopers and NECX. The expansion is expected to escalate through October as new offices are added to the corporate fold. HookMedia is funding the expansion through $15 million in venture capital raised in March.