There is a clause in every book contract that says the design (and some other things) is the sole preserve of the publisher. You should breathe a sigh of relief, as book design is a tricky thing.
Design is most obviously complicated in the case of an illustrated book. It's not just a case of picking the right illustrator or choosing the right photos. Pick up an illustrated book. Ignore the content of the pictures. What has the designer done? You need to look at where the pictures are on the pages, what size they are, how they relate to the text. Do they bleed off the edge of the page or are they surrounded by white space? Is text integrated into the pictures? Does it flow around them with a curved or jagged line, or is everything in rectangular blocks? How is white space used in and around the pictures?
Now look at the pictures themselves. How does the palette of colours used in the pictures work with the colour of the paper and any colour in the text? What does the style of the pictures tell you about the book? Are they whimsical, flippant, strong, aggressive, traditional, avant garde?
But design is important in even an unillustrated book. The design tells you a lot about the type of book you are reading. If the type is small and cramped and the margins are small the book is probably printed on thin, off-white paper and is most likely a cheap novel. It might be a mass market paperback bodice-ripper or a cheap edition of a classic. If there are large margins, lots of space between the lines (extravagant leading) and a font that is airy and leaves lots of space (long ascenders and descenders) you are reading either a book for reluctant readers or an expensive poetry book, or possibly a hardback novel which is shorter than you feel it should be for the money you spent. There are plenty of other elements of design: the running headers and footers; whether chapters start on a left- or right-hand page; how much space there is above a heading or chapter head; the font(s); the leading, the kerning of the title and headings; how many pages the book runs to ...
And I'm not even going to talk about the cover.
Book design is a specialist art. Good book designers make a book a thing of beauty. And book design is one of the reasons that e-books are currently inferior to real books. The look of the text is ugly, and that's a shame.
The designer rarely gets a credit on the acknowledgements page of the book, but they should. Their work is more visible to the reader than that of the editor or the proofreader. A useful trick for writers struggling with a book they can't evaluate is to print it out in a different font. Text looks different in a different font. The book you wrote in Arial (yuk) or Times New Roman (yuk) will look quite different if you print it in Goudy Old Style or Palatino. Suddenly, you can see the book with fresh eyes. Now - does your book suit a spindly, spidery font or an elegant, crisp font? Does that curly 'Q' really work? Do you have to change the font because of the style of one letter? (I often do.) *Now* you see what the book designer does.
PS If you can't see any difference, please leave the blog directly, do not pass Go, and do not collect £200 (as if!).