The premiere weapon that came with Whispers in the Walls in Warframe was the Grimoire. A whole new weapon class that is admittedly very strong and fun to play with. Alongside it, there were a whole slew of new mods that you can use with it exclusively. There is going to be a lot of tinkering with this new weapon as time goes on, but here is the build I have to work best in the new content so far.
A Proper Grimoire Build in Warframe
The Grimoire build I have been using is as follows:
For starters, the Creeping Bullseye seems detrimental for rapid fire, but if you use a wisp, or volt. You can offset that reduced fire rate and keep the high critical chance. I prefer using radiation since it is effective against armor and corrosive also strips it to make things easier to kill. Finally, the Xata Invocation mod is seriously helpful to maintain a steady flow of energy for your warframe. If you forma it any further, do try and go for primed target cracker instead. The grimoire with this build is very useful, especially in the new activities.
But this is the best build I can come up with for general use in Warframe. There are other tome mods that can suit an array of different warframes, but this build is more general purpose. I hope it helps you!