We've created this Forum Thread with some workarounds and advice to assist you in publishing you ebook.
Print-ready file resolution looks different than the uploaded PDF
Created a 244-page book using InDesign with several grayscale images. Made the PDF using Lulu's PDF preset. The file looks sharp. After Lulu creates the Print-ready file my fonts no longer look sharp. Body text is Adobe Caslon Pro and san-serif text is Myriad Pro. All fonts are embedded in the PDF file.