Book Module: How do I get automatic captions on each page?

Setting this up at first can seem a trial, but once you have defined the Presets/Templates then it becomes simpler.

You need-

1)  A Custom 'Photo Text' Preset

Click the Drop-down menu and - 'Edit' adding the information you want below the photos-

2) A Custom Page Template

After setting one page with a photo and the Custom Photo Text- [Right-Click] the page and "Save as Custom Page".  (Unfortunately you cannot give names to Custom pages)

3) An 'Auto Layout' Preset-  that uses your Custom Page Template.

Click on the Auto Layout Dropdown menu to 'Edit Auto Layout Preset' and choose the Custom Page-

Check the box for [Add Photo Text],

**Now (IMPORTANT) Click the top Preset box for the drop-down menu and [Save Current Settings as a new Preset]

You now have a complete Layout Preset with the Page design & Text that you can use to 'Auto Layout' your book.

Even better- if you always want this same Layout- set the Preset once in the Auto-Layout panel (it becomes the 'sticky' default until changed), and set your Book Preferences to 'Autofill'. 

Now you can select a folder/collection of photos- Click the 'Book' module menu- Done! a complete book with your Custom layout of photos and Photo Text.  That's ONE CLICK OF THE MOUSE!!!

Here's a video that explains how to add specific metadata fields and custom text to accomplish your objective.

https://www.youtube.com/watch?v=pNnr2kYrZUo

I guess one way to approach this (for simple books that you want to PDF) is to just have your selected set of images, and use a template in the print module (where custom caption templates work very well) and just select all the images and print to PDF. You get nothing but a pile of pages with the images, but if that's what you're after, it works just fine.