How to fix lost connections to original high-resolution pictures in iPhoto and how to import an iPhoto Book from one iPhoto library to another

Question

Hi there,
I am using a MBP Pro with 8 GB cache and 1 TB HDD from beginning of 2011 with OS X Yosemite and iPhoto version 9.6 (910.29).
I created an iPhoto Book to print which contains my best pictures of 2013 and 2014. Then I got an error message when I wanted to print the book, saying (translated from German):
Book could not be created.
With photo p233432343.jpg there was an error. The original version of that photo with full resolution could not be found or is damaged. Please replace the photo or delete it from your book.
When I enquired further, I found out, that many, many pictures in my iPhoto library have that problem - only the thumbnail picture is found by iPhoto, but not the one with the high resolution. When looking at the photos in detail one sees that they are not sharp because obviously the photo book only refers to the thumbnail. When I went into the iPhoto Library using Finder, I saw that the original high res photos are still there. As there are an estimated 3000 photos with a lost connection (some still have, it but about 1/2 of my photos don´t), it does not make sense to import those photos manually. This photo book is just the tip of the iceberg, so to say.
Then I looked for solutions in some forums. I had two lPhoto libraries, one on my HDD (120GB) and one on an external HDD (175GB). Assuming there might be problems, I have deleted (moved) the one on my HDD, because the "right one" is the one on my external HDD and started iPHoto again using the alt key. But that did not work either.
I then tried to repair my iPhoto library using iPhoto Library Manager, and merged and rebuilt libraries. The library that came out was crap - all folders that had lost connections to the high resolution pictures now have zero photos in it. Also it is lacking the projects, including the photo book I created.
Then I went to find out if there is a way to import the iphoto book project from the old, corrupt library into the new, repaired library but could not find anything. It can only be exported as a file structure or website. As the file structure is missing the high res pictures, that does not even solve the "little" problem of the photobook.
I would REALLY appreciate your help on the two questions:
1- Can you advise what I need to to do to to get the connection to the high resolution photos established again  ? As it is more than 3000 photos, I use an automated way of reestablishing that connection. Probably an answer to this question would make question 2 irrelevant.
2- Do you know if there is a way to export and import a book project created in iPhoto from one library to another ?

Answer 1

1 - Select  the thumbnail of one of the problem photos and use the File ➙ Reveal in Finder ➙ Original menu option.  This will take you to the folder containing the original file with that file selected by the Finder. 
You can also use Find Any File to search for that file (get the file name from the Info pane in iPhoto). You will find the thumbnail version and maybe an edited version also.  Made sure what is found is the large, original version and note were it's located.  Search all of your drives with FAF just in case.
If you don't fine the full sized version then the only recourse is to use a file recovery application like   SubrosaSoft FileSalvage on your drives to see if any of your full sized photos can be recovered.   If you find enough files to make the purchase of the software worthwhile buy it and recover the files.  However, since it may have been quite a while since you discovered the missing originals it seems unlikely you'll be able to recover any due to being overwritten.
2 - book projects are just entries in the library databases and can't be exported to another library.