Hey RPFers, I've just started the grail quest to build a diary aswell. Here's a top tip that no one seems to have mentioned here yet. The original source books that were used by the prop master Keir Lusby are:
The Grail: Quest for the Eternal: Quest for Eternal by John Matthews

The Flowering of the Middle Ages by Joan Evans

Palimpsests : The Backs of Monumental Brasses by John Page

Books of Hours and Their Owners by John Harthan

A History Of Architecture On The Comparative Method - Sir Banister Fletcher

The Reliquary & Illustrated Archaeologist by J. Romilly Allen
The first two are the ones to get, as they have the majority of the images that were used, but there are some sections in the others also used according to people on Club Obi-Wan. I've just ordered the lot and once I've made some high res scans of the source material I'll happily share it to stop people needing to hunt down the books.
Another great source for the original pages is a book called From Star Wars to Indiana Jones : The Best of the Lucasfilm Archives, which contains good quality photographs of the original hero prop from the Lucasfilm archives. As I find more sources I'll happily include them in this post to hopefully help people along the path.
Mike
Edit. Some good online resources are obviously zenseeker although it does have lots of dead links as previously mentioned, it really is a great compliation of all kinds of resources and has a lot of the detail that you're looking for, but it lacks the photos of the individual pages so it's really challenging to figure out exactly what it's talking about
Club Obi-Wan has some great stuff, including a list of all the inserts and a list of all the pages and their order, but again lacks any kind of high resolution images of the original prop pages (as far as I can see). However it does have lots of images of other fan created diaries, that can be great material to work from.
Indy Magnoli's website has some good low res images that'll help you figure out what each page looks like, though they are too low res to really read and recreate the pages directly from.
Propstore had one of the Hero Diaries in 2010 on display at San Diego Comic Con, which they scanned and put on an iPad. There's some poor quality images of those pages in this video:
Propstore recently sold a B Diary, and so they've got some good quality photos of it's cover and some interior pages:
Adam Savage on Tested did a video going through this diary:
Propstore also sold some individual production pages:
Venice Library Pages:
INDIANA JONES AND THE LAST CRUSADE (1989) - Production-Made "Venice Library" Grail Diary Page
Grail Cup Pages:
INDIANA JONES AND THE LAST CRUSADE (1989) - Production-Made "Grail Cup" Grail Diary Page
Two Map Pages:
INDIANA JONES AND THE LAST CRUSADE (1989) - Pair of Production-Made "Maps" Grail Diary Pages
Knight Crossing Invisible bridge pages:
INDIANA JONES AND THE LAST CRUSADE (1989) - Production-Made "Knight Crossing Invisible Bridge" Grail Diary Page
Adam Savage on Tested did a video going through these pages:
Propstore also sold a job lot of photographs of the original hero prop, which they've uploaded low-res images of:
p classl1desctextwhitespan stylefont-weightboldThis lot will be auctioned on Wednesday, June 30th. The auction will begin at 900am PDT and lots are sold sequentially via live auctioneer tune in to the live streaming broadcast on auction day to follow the pace. Note other lots in the auction may...
propstore.com
This is a great thread with some good info about inserts:
Hello, As many other already did, I want to create my own version of the grail diary, based from what can be available, using as many old and original items as possible, even not accurate to those used in the movie, for exemple, the french nott won't be a replica, but a real french note, same...
therpf-f28a.kxcdn.com
Perhaps we should all work together and create some kind of PDF full of reference material, like the
Lightsaber Guidebook