Textbooks are written by a committee to appease elected school board members and educational committees to be sold to students that have no choice and zero unmotivated interest in the subject. It is perfectly reasonable to find a textbook about biology with no chapter about evolution because Texas won't buy it if it has one. College textbooks aren't much better - they are a catch-all overview of a subject as described to eliminate all controversy whose narrative is sculpted in such a way that retention of key facts and figures can be interrogated at regular intervals for the purposes of easy grading.

There is nothing readable about a textbook. There is nothing in a textbook that will draw you in - to the contrary, a textbook is designed to shove an arbitrary, selected-by-committee period down the throats of people who have no choice but to read it in the amount of time allotted by another group of professionals that don't give the first fuck about the material or its importance.

A nonfiction book, on the other hand, is one author's exploration of a subject following her narrative and what she thinks is important, using her voice to explain and elaborate on facts that you might otherwise find uninteresting. Here, check it:

“So gorgeous was the spectacle on the May morning of 1910 when nine kings rode in the funeral of Edward VII of England that the crowd, waiting in hushed and black-clad awe, could not keep back gasps of admiration. In scarlet and green and blue and purple, three by three the sovereigns rode through the palace gates, with plumed helmets, gold braid, crimson sashes, and jeweled orders flashing in the sun. After them came five heirs apparent, forty more imperial or royal highnesses, seven queens – four dowager and three regnant – and a scattering of special ambassadors from uncrowned countries. Together they represented seventy nations in the greatest assemblage of royalty and rank ever gathered in one place and, of its kind, the last. The muffled tongue of Big Ben tolled nine by the clock as the cortege left the palace, but on history’s clock it was sunset, and the sun of the old world was setting in a dying blaze of splendor never to be seen again."

That's the opening paragraph of Barbara Tuchman's The Guns of August, a book about WWI and its causes. If that shit doesn't give you frisson, if it doesn't get marshal music playing in your head, if it doesn't make the Game of Goddamn Thrones leap to the forefront of your imagination you're dead inside. It sure as fuck makes you want to keep reading.

Compare and contrast:

There aren't a lot of college courses that cover WWI explicitly. AP World History classes give it a chapter. Lewis & Clark College, a decidedly liberal arts place, uses The Oxford Illustrated History of the First World War. The most popular AP book appears to be World Civilizations: The Global Experience.

Neither book so much as mentions Edward VII.