I am a little confused by the statement. Does it suggest mobs spawn near obsidian, or that once spawned do mobs favour moving towards obsidian? In both cases, here are my own observations from playing over the past few years:
I tend to find obsidian naturally where lava and water meet. In an underground cave system the light from the lava appears to stop mobs spawning directly near the lava/water boundary. Furthermore, I have never witnessed mobs willingly move to group up on the obsidian. This would suggest that mobs are not attracted to obsidian (either through spawning or moving).
I only place obsidian to create nether portals. In this case I have noticed zombie pigmen spawning regularly in the overworld. I haven't really noticed the same in the nether (i.e. zombie pigmen just seem to spawn anywhere in the nether).
For a definitive answer I guess you could reverse engineer the spawn and movement code. As you have already noticed, there is nothing really to suggest through other sites that obsidian attracts mobs. I would be surprised if the book's statement turned out to be true.