Monroe and Rosalee couldn't be replaced by books. They didn't just provide information but gave Nick credible sources into the wesen world. A book just doesn't work the same way. Their insight and opinions on a particular subject mattered a great deal.
More importantly, Monroe and Rosalee actively took part in plenty of Nick's cases. Who saves Hank from the Siegbarste if not for Monroe? Who rescues Juliette from one Damonfeuer while Nick fights the other? Who helps out with the carnies if not for Rosalee? Who helps come up with a solution to Portland being overrun by zombies?
Most of the people Nick works with aren't in the know. That's why Monroe is often chosen to go undercover. Rosalee has done the same. It's their character more than their knowledge that's so important.
As for Juliette and Adalind, a unique set of circumstances got them to where they are now. The issue of Adalind being with Nick was caused by a bunch of things that were unforeseen in-universe. Nick is with her now because of Kelly in combination with Juliette turning to the dark side. Even Kelly alone wouldn't be enough to put Nick and Adalind together. I know the writers can do whatever they want but them being paired together just because Kelly brought her to Portland to stay with Nick wouldn't go over as well. It was Juliette's betrayal that helped bring them together.
More importantly, Monroe and Rosalee actively took part in plenty of Nick's cases. Who saves Hank from the Siegbarste if not for Monroe? Who rescues Juliette from one Damonfeuer while Nick fights the other? Who helps out with the carnies if not for Rosalee? Who helps come up with a solution to Portland being overrun by zombies?
Most of the people Nick works with aren't in the know. That's why Monroe is often chosen to go undercover. Rosalee has done the same. It's their character more than their knowledge that's so important.
As for Juliette and Adalind, a unique set of circumstances got them to where they are now. The issue of Adalind being with Nick was caused by a bunch of things that were unforeseen in-universe. Nick is with her now because of Kelly in combination with Juliette turning to the dark side. Even Kelly alone wouldn't be enough to put Nick and Adalind together. I know the writers can do whatever they want but them being paired together just because Kelly brought her to Portland to stay with Nick wouldn't go over as well. It was Juliette's betrayal that helped bring them together.