This seems strange because exporting and printing should be different things. Are you by any chance trying to PRINT TO PDF instead of exporting? If so, just don't!
No, I was not trying to PRINT TO PDF. I was trying to export the book to PDF by following File > Export... and then selecting PDF for the output. That didn't work. However, I just realized that I can export the book by clicking the little down arrow at the top right conrner of the menu that shows all the documents of my book (I don't know what it is called). From there I selected 'Export Book to PDF...' and voila! It worked.
You mentioned: . . . and I am unable to see any missing fonts or images on the computer screen.
You cannot simply scan your document onscreen and assume that since you don't see any images or text missing that the file is ok.
These warning message need to be resolved to make sure your document will print correctly.
Images are linked when you place them, and if they somehow become unlinked, they will be reported as missing.
InDesign will still show a low resolution preview of missing images, but they will print in low resolution using only this preview rather than the higher resolution of a properly linked images.
To check your images go to the Links Manager. You need to relink any images with broken/missing links.
When fonts are reported as missing, that doesn't mean that text will actually disappear,
There will be some font substitution and the substituted font may not match your original font.
This may cause overset text and other issues related to the differences in fonts.
Check for and replace missing fonts using the Find Font dialog.This will show all fonts in your document and show any missing fonts.
Note that in addition to fonts of text in InDesign, it will also show missing fonts in any images that have live text layers.
If fonts are reported as being in graphics and missing, you will again have font substitution when you print. This image may appear ok on screen, but it won't print correctly.
Thanks for the note. I have now been able to re-link missing files which were line works from Illustrator and so seamed almost perfect on the screen.