Adding veggies to bread

Agree with Lori about being careful not to inadvertently add more water/liquid than needed when adding vegetables. Also:
I often knead solids in for flavor/color when preparing the dough for the second rise. That way, the additions don't interfere with the chemistry of the yeast-sugar-water-salt if using. And I can get a better sense (both visually and by weight/texture) of how much I want to add.
If you're using a single ingredient, it's sometimes fun and useful to put a slice or piece of it on the top of a loaf a roll just before baking, so when done it indicates what's inside.
See King Arthur site (which Lori also referenced) for many good recipes incorporating vegetables into bread.

I used to make a version of veggie bread when my kids were younger, and less inclined to eat them. I can't tell you exact measures for a single loaf, but through trial and error I figured out that I could include up to 4-5 cups of grated or finely minced veggies per 4-5 cups of flour, and make two loaves. I also used half whole wheat flour and half bread flour. Zuchinni and carrots can be used shredded and raw. Use part of the salt called for in the recipe you use to lightly salt the zuchinni, and let that sit in a strainer for a while to help remove excess water. Then squeeze as much out as you can get. Carrots don't need any extra prep. Spinach does best if you cook it until limp, and also squeeze out extra liquid. If you use spinach, you can also use the water you remove from it to add into the bread as part of the liquid. I would start with mixing the veggies into the wheat flour, yeast, and the liquid called for in the recipe- about 1 1/4 cups to start. Once that makes a rather wet batter, I would stop the machine and let it sit to rehydrate the wheat flour for 20 minutes. After that, I kneaded in enough of the bread flour to make the usual bread dough consistency. You can also add in minced dried tomato, either in oil or not- and even use tomato paste, diluted in the liquid called for in your recipe. Tomato paste bread with basil and onion flakes makes a wonderful BLT, or grilled cheese sandwich by the way. I did not notice that any extra yeast was necessary, honestly. The major thing to watch is the amount of liquid your veggie will bring to the dough. Extra watery stuff like squash or spinach need to have some of it removed. You can add in whatever dry herbs or even minced garlic, pretty much whatever you like, though I would say not more than a tablespoon combined. As far as a particular recipe, I a basic white/wheat dough recipe from King Arthur as the base dough to start with. I have also made veggie bread using dehydrated veggie mix, which I rehydrated with boiling water to soften and then cooled to room temperature. My kids were all teenagers before they discovered that some of their favorite breads were veggie loaded. One nice thing about adding veggies to bread is that it slightly increases the amount of time it will stay fresh. If you have a favorite bread recipe already, I'd say use that as your starting point, and introduce the veggies you'd like to try- maybe a cup or two, before you add in much liquid. If you do that, and add in liquid needed to form the dough- you shouldn't have any problems.