"Less has always been used in English with counting nouns."
Over the years I've changed from prescriptivist to descriptivist. Surprisingly because once I started to teach English my eyes opened to the vast discrepancy between formal grammar and actual use. Eventually I began to realise that language is what people use, not what people tell you to use. I still correct people when they are learning English but I'm only strict where it interferes with meaning. Using less instead of fewer certainly does NOT confuse the meaning.
[1] https://en.wikipedia.org/wiki/Fewer_vs._less
Edit: jake-low beat me to it - I'll leave the link though
Of course this situation is manifestly unfair to ESL speakers, or anyone whose lack of familiarity with formal English leads to misconceptions about their intelligence.
There has always been bad grammar too. Also forms that were once fully correct can become incorrect. I try (with mixed success) to stick with whatever pedantic people have claimed was the rule for the last 100 years or so.
Using "less" instead of "fewer" is not very damaging. But when style guides consistently say "you can do this, or that, it's up to you" then the language just becomes more ambiguous and opaque than it already is.
A good example is allowing "phenomena" as a singular. This just creates ambiguity. Now if people had started to use "phenomenons" as a plural, there might be a reason to rubber-stamp it.
Personally, I read the title to be a pun on "30 minutes or less", a phrase American pizzerias sometimes use to advertise guarantees about their delivery times. The book is promising to show you useful programs in "500 lines or less, guaranteed!".
I parse "less" as "less code" and not "fewer lines".
[0] http://bitemyapp.com/posts/2014-11-22-literate-url-shortener...
