I have read John Grisham a lot, 16 or so novels from 2012 up to now (it´s a bit lawyerish, but the guy have some interesting story-telling skills).
Then I have read David Baldacci, Alvin Toffler, Agatha Christie, Adam Hall, Allison Pearson (a more comtemporary author), Alexander McCall Smith, Burt Hirschfeld (Fire Island - a novel), Tom Clancy, Candace Bushnell, Aron Ralston (127 Hours - Between a Rock and a Hard Place ) I´ll read when I find it in my messy things, Carmen Bin Ladin, Cecily Von Ziegezar, Colin Forbes, Colleen McCullough, Chip Heath/ Dan Heath, Daniel Silva, Daniele Steel, Dean Koontz, Eric Hobsbawm (The Age of Extremes), Elizabeth Gilbert (Eat, Pray, Love), George Orwell, J.K. Gailbraith (Money whence it came, where it goes), Joseph McNamara (The First Directive - a novel), Irving Stone (Depths of Glory - a bio of Camille Pisaro - a painter), Ken Follet, Michael Crichton, Vikas Swarup (Slumdog Millionaire), Penguim Mini Books (mini version of Beowulf and of The Ilíad.), Tom Clancy, Terry Pratchet (Going Postal - a funny and entertaining fiction book.), Stieg Larsson, Francis Roe, Orhan Pamuk...
Those constitute some thirty percent of those that I read (ecletic bunch, eh?), and I have more than a novel/book from some of them. Not to mention companion books, some about English Idioms, textbooks, grammar, and sites, etc.
So, I am not the guy to answer your question fairly, everything English seems like entertainment to me! but I admit that books can put many guys to sleep!
Anyway, should I have to give you one answer it would be, entertainment books can be a tool to train your English and build up some vocabulary as well. But then, you have to use some of your spare (or sometimes not so spare) time, it have to be of your liking and it depends on your learning style. Or even your prefferences or pocket size (granted, I really have bought second-hand books/novels).
One can learn English in a lot of ways, believe me; reading novels, entertainment, etc, is only one of them.