Just a little question. According to the HH novels Erebus became a personal advisor for Horus and manage to manipulate him to treason. That made me think. Were the Luna Wolves/Sons of Horus the only Legion that got infiltrated by the Word Bearers like that, or did they send their "agents" to other Legions as well?

I don't think there's any evidence either way--I'd assume Erebus went to Horus because he was the Warmaster, so if he could turn him then he'd have got straight to the top.

Yes, but Horus still needed to get those on his side who potentially resented the fact that he became Warmaster. I don't know how he managed to convince Pertubaro and not Rogal Dorn, especially since Dorn was more of a brother to Horus (as in closer) than his Iron Warrior counterpart.

I think a lot of it comes down to situation - all of the Primarch's who sided with Horus had the basis of their heresy in some sort of emnity towards the Emporer. But, as Magos says, I think the key was still getting to Horus and in doing so creating a rival leadership figure to the Emporer. I think of it as something similar to many of the historical fueds between noble families in europe, where different factions would ally themselves behind a particular royal aspirant and support them in their bid for the throne.

In the case of Perturabo, his descension had been growing for some time. Apparently he was a massive genius who used his knowledge to make many of the more difficult invasions of the crusade take place, but the kudos often went to his more glamerous brothers who actually spearheaded the attacks (I remember reading that there was a specific example of this taking place with dorn). His legion was often tasked with the most drudging of garrison and defense duties and gradually spread out. When Olympia, his home world revolted, I think it was the straw that broke the camels back and cast the entire nature of the crusade into doubt (after all, if your home world could revolt, then what chance is there for other places?) Horus, who was rival to the Emporer in terms of his charisma and leadership, offered a stronger and more secure future for Perturabo which would restore and restate the value of him and his legion. All in all, I would say that his decision to side with Horus was pretty understandable, as by all accounts he had been pretty poorly treated during most of the crusade.

I think it quite likely that the Word Bearers had representitives sent to other Legions as well. The Chaplain Edict charged the other Legions with emulating the Word Bearers practice as regards Chaplains, so there would be a perfectly reasonable reason for the Word Bearers to send representitives to other Legions to assist them in introducing their own Chaplains. Indeed, many Legions may even have requested them to help with the training of their Chaplain departments. Once in place, they could help work towards the corruption of the Legion, either as advisors, by planting seditioous ideas, or by encouraging the spread of Warrior Lodges. Seems very much in character for the Word Bearers to be honest.