What makes a book easier or harder to read? I am interested in books by academics that examine the following. How to Read a Book: The Classic Guide to Intelligent Reading is the only relevant one that I found.

Suppose a reader to be deciding which books to read about a topic. Why do some readers:


judge some books easier to read than others? 
comprehend some books faster and retain more than others?
What do authors (not) do that affect 1 and 2 above?
This question is open to some amount of opinion as there can be differences due to education level of the person reading the material etc.  However, there was a study by the American military in order to make their technical manuals as easy to read as possible.  The study was started by Rudolf Flesch with his Reading Ease evaluation, known as the Flesch reading-ease score (FRES) test, and Flesch later worked with J. Peter Kincaid to develop the Grade Level evaluation for the United States Navy, known as the "The Flesch–Kincaid" (F–K) reading grade level or Flesch–Kincaid readability test (Kincaid, et al. 1975)

Using the total number of words, syllables and sentences in the text, the formula for the Flesch reading-ease score (FRES) test is

$Score = 206.835 - 1.015 \left(\frac{total\;words}{total\;sentences}\right)-84.6\left(\frac{total\;syllables}{total\;words}\right)$

The readability tests used by schools is the F–K reading grade test.  The "Flesch–Kincaid Grade Level Formula" instead presents a score as a U.S. grade level, making it easier for teachers, parents, librarians, and others to judge the readability level of various books and texts.

Using the total number of words, syllables and sentences in the text, the Flesch–Kincaid (F-K) Grade Level Formula is

$Grade = 0.39\left(\frac{total\;words}{total\;sentences}\right)+11.8\left(\frac{total\;syllables}{total\;words}\right)-15.59$

Scores can be interpreted as shown in the following



References

Kincaid, J. P. Derivation of New Readability Formulas (Automated Readability Index, Fog Count, and Flesch Reading Ease Formula) Memphis:Naval Air Station Memphis [US Navy Unclassified PDF]
Available at: http://www.dtic.mil/dtic/tr/fulltext/u2/a006655.pdf