I believe that malted barley promotes the bread rising, while barley flour (just ground barley, unsprouted) will affect texture. In my experience the barley flour results in a finer crumb, more moist outcome.

Actually, I checked on the 'net and many malted barley flours are straight diastatic malt. In this case the most you want to add is 1/2 to 1 teaspoon per loaf.
If you dissolve the flour in some of the liquid in the recipe and heat to near boiling you can destroy the enzymatic activity and use it only for flavour.

You need to ask if the malt in the flour has been raised to a high enough temperature to inactivate the enzymes in the malt - ie if it is non-diastatic malt. If the flour contains diastatic malt it is certainly possible to add too much and end with damage to the dough.

I only use King Arthur All Purpose for all baking, and it's already got malted barley flour in the ingredients. How much, I don't know. Might be on their website somewhere.

I always buy it in bulk at The Food Mill in Oakland. Amazing flours. Amazing everything, actually. I've never bought barley flour anywhere else. (I take it with me when I travel!) I have no idea who their supplier is, but if I can find someone who knows when I go there next (should be in the next week or ten days), I'll ask. No doubt it's a local mill that supplies the best bakeries, pâtisseries and restaurants in the area, if The Food Mill doesn't grind it themselves. ;o)

Thanks guys. AntoniaJames, which type (and brand) of barley flour do you like when baking you bread? It would be great to hear other people's experiences with the different types as well.

Malting is the process by which the grains are germinated by soaking in water. The grains are then dried and used accordingly. This converts the starches into sugars. For example, you have to malt grains to make beer, or wort for whisky. Can't have a fermentation without sugar. So malted barley flour will have less starch and a sweeter flavour. . . .a malt flavour.

I know there are others out there in foodpickle land who know a lot more about this than I do, but I read somewhere (probably on The Fresh Loaf, when I was testing a dinner roll recipe last month) that malted barley flour is actually already in most all-purpose and bread flours sold in the US. Too much of it will damage the dough. Or so they say. I would not, under any circumstances, substitute one for the other. I'd probably go back to The Fresh Loaf and just search for malted barley flour, as I remember there were quite a few threads. ;o)