Adding Pumpkin Puree to a Recipe?

Hello, Fellow Bakers! First time here. :)

I am looking to add pumpkin puree to a yeast bread (a type of doughnut, to be exact) and am wondering how to go about doing this? I have noticed that most recipes also include some oil - is there a reason for this? Here are the ingredients for my recipe:

2 eggs, room temperature
1/2 C. milk, room temperature
4 tbsp. butter, melted
1/3 C. granulated sugar
1/2 tsp. vanilla extract
1 tsp. salt (I would recommend using a bit less - perhaps ½ tsp. - if you’re using salted butter)
2 C. + 2/3 - 1 C. all-purpose flour
1 tbsp. active dry yeast

I always do an internet search to try and find the answer/s I'm looking for, but for some reason I've had a hard time finding info of this sort. :/ (Also, as a side not, does anyone know of a good, comprehensive book that covers the science of baking that would help someone like me (a blogger) to better adapt recipes?)

Any help would be greatly appreciated!