Take a look at the Iron Kingdoms Character Guide. It was released waaaaay late, but it's flying off the shelves. (Several of the LGS's in my area have found it to be one of their best selling books this year.)
I think it all comes down to the type of book. Penumbra Fantasy Bestiary got hurt by not being a 3.5 monster book following the change to 3.5. Monsters are something that will greatly benefit from the most updated rules and they should have retooled and put it out even later. (Although, I bought it and love it, even if it is 3.0 and I play 3.5.) It wouldn't have flown off the shelves, but it would have fared better. Something like the IKCG was going to sell regardless. 3.5 information for the races and equipment was icing on the cake. It's a hugely popular setting not only from the Witchfire Trilogy and the Monsternomicon, but also from the minis wargame. I'm just glad they took the time to get it right rather than gives us a product that was truly finished.
Kane