The interior design is a vital part of your book, and must work with the cover design to invite potential readers into your world. The interior must reflect the story, inviting readers into your world, and helping them to visualize the landscape.
If the inside of the book doesn't make it easy to read, and to follow the story, your book is defeated before it can begin. Margins should be a comfortable distance from page edges, fonts should be easily legible and should be a reasonable size, neither so small as to need to strain to read it, nor so large as to look like a large-type book, unless that is it's purpose. Most novels also benefit from having chapter titles on each page, or chapter title and author on facing pages. Non-fiction books might also benefit from having section titles on pages, to help readers find what they're looking for. A table of contents for novels, and both a table of contents and an index for non-fiction is also helpful - imagine looking for information in a 500-page book, with no index to easily find it. While this part of the book's design can be tedious and time-consuming, it's a job that's worth doing well.
Speaking of the index, the setup in that area is also very important - headings, sub-headings and sub-sub-headings should be clearly set apart so it's easy to follow.
It's never too early in the process of writing your book to discuss the interior design - you know how you want your book to present itself, and it's my job to work with you to make it happen in the most professional way possible.
If you'd like to discuss your project with me, please email me with contact information and I'll get back with you as soon as possible.