What kind of relationship was there between Philippa and Triss in the books? (The Witcher 3: Wild Hunt)

None, just the majority cannot read. The dreamers refer to a passage where, during a megascopic connection between Yennefer and Triss, Triss finds herself in a room with Philippa. Both Triss and Philip are in clothes, there is not any hint in the book of the novel in this passage. It is quite logical that those who have lost the trust of the kings of magic are forced to hide, so Triss is hiding in Philip's castle. And in fact, where else could she be ...

Exactly those that both are members of the Lodge of Sorceresses.

Swallow Tower. Chapter 9.