There's been some interest about how to do this, so here's a little tutorial. It does take time but it's a pretty straightforward process once you know how to do it!
1. Flatten your images and save as JPG files, reducing their image size. As the book is meant for online viewing and not printing, I change the resolution to 72 DPI and also change the size to 8x8.
2. You can speed up the process by saving the above as an action and batch process the layouts. This book of mine has 122 pages but it took me only 5 minutes to convert all of them to JPG. Make sure to check "include all subfolders" if your layouts are not in the same folder.
3. Arrange your pages. I like my digital book to resemble my physical one, so I order my pages the way they appear in the actual book. I put the number 01 before the cover, 02 on the first page, and so on. Note that I also made a blank page between the cover and the first layout, to resemble printed books where the first page always appears on the right hand side. This blank page is designated 02Blank.
3. After you've all your pages numbered in order, you need to merge them into one big PDF file. This can be done in Photoshop: go to File > Automate > PDF Presentation... In the window that pops up, click "browse" to find your files. Add them in order. Your numbering system should allow you to do this fairly easily. However, if your book has over 100 pages, Photoshop will try to place page 100 between page 10 and page 11. To avoid this, first add page 01-99, then add the rest of the pages. Also, make sure that "multi-page document" is selected.
4. Once you have your PDF file merged, it's time to upload it and get ready to publish it. I use issuu.com to do this. You'll have to sign up before you can upload. Then, select upload and you should see this screen.
5. Drag your big PDF file onto the screen and it should start uploading! Depending on the size of your file, this may take a while. When you're done, you'll be able to edit the title, add a description, and publish the book!
6. Check to see if everything looks the way you want it to, then share it to your heart's content! I like using their sharing feature so my friends and family can flip through the book right there on Facebook!
And that's it! I hope it's easy to follow. Let me know if you have any questions!