NOTE: This update is for the previous version of the book (which is free) and not for Windows Phone 8.1 Field Guide. --Paul
Here's an update to the earlier (and free) Windows Phone 8 Field Guide which adds PDF bookmarking so you can navigate through the various headings in the book using the Bookmarks feature in Adobe Reader or Acrobat, or whatever PDF reader you choose. I updated the MOBI and ePub versions as well, though I didn't change any of the content (and those versions already support that kind of navigation).
If you're curious how this works, I updated Windows 8.1 Field Guide last night. So check out the post Windows 8.1 Field Guide 1.05 on that book's web site for the details. Future updates to Windows Phone 8.1 Field Guide and Xbox Music Field Guide will feature this functionality as well, but I don't see any reason to do so now since they're both in-progress.
Note: Remember, MOBI format works on Kindle and ePub works on Nook.