Just off the top of my head I can tell you that Platinum features different wild Pokémon, different Gym Leader teams (and two Gym Leaders actually switched orders, so different levels, too), different Elite Four teams, and an entirely new area as part of the main storyline that isn't present in DP. And I think there are some movepool changes from DP to P, too, so if your guide has the PokéDex in the back, that may be off as well. So while a Platinum guide may work fine for very generic information in Diamond, it's not really going to help much when it comes to specific aspects.
But if you'd rather not buy another guide, you can use sites such as Bulbapedia for getting information.