I emailed customer service about this as I have the same problem - typical get to book 10 in a series and it's not available for download even though it has previously been possible to buy for Kindle! They said: "Although the book was published on Mar 27, 2012, it has now been removed from the website. Occasionally books are removed from the Kindle Store for various reasons. We don't have any details about why this particular book may have been removed. At this moment we can't confirm when this book will be available with us. Perhaps you could check our website from time to time to see if this book is listed again for sale. I have also forwarded your comments to our business team, to let them know that you are interested in this book." I have since downloaded the book from iBooks onto my iPhone and was able to get it no problem for the same price. I guess it's either that or else purchase the hard copy of the book if you want to read it any time soon.
Yeah I've noticed this problem too, pretty annoying since it once again lists a price and says availiable for download now. Maybe it's time we all email amazon begging and pleading for it back. Lets just hope they release it again soon.