First let me say I really enjoy “The Bread Bible.” I am new to your forum and have a question regarding the recipe on page 226 for the Potato Sandwich loaf. Is the liquid for the sponge (1/2 cup) correct? The sponge was very heavy and did not mix well with the flour mixture. I added more potato water to the mix and the loaf finished beautifully.

Glad the loaf turned out! I did check the errata published over on the blog section, and didn’t find any listed for this recipe.  You could post a question to Rose here: http://www.realbakingwithrose.com/2006/02/corrections_the_bread_bible.html

It does look as though the sponge is a thicker one. According to the pre-ferment section in the beginning of the book, Rose’s typical sponge uses equal volumes of flour and water, and this sponge has a little over 3/4 cup flour to 1/2 cup (including honey) liquid.  The final hydration also suggests a firmer dough, 63%. 

A few things to check:
-you didn’t include the mashed potato in with the sponge, did you?  It is prepared in the sponge step, but added to the dough later.
-are you weighing flour and water?  With a thicker sponge, an inaccurate measure of flour or water could tip the balance of ingredients so that what was intended to be a thick sponge becomes an unworkable one.
-are you using unbleached AP flour from one of the brands listed (not bread flour)?
-did you cover the sponge/blanket with plastic wrap during the pre-ferment?
-did you include the butter?

Here’s a link to a photo/write-up of Marie Wolf baking this bread:  http://breadbasketcase.blogspot.com/2006/10/potato-sandwich-loaf.html

She didn’t mention any issues with the sponge, but at least you could see her texture, etc.