As teachers we can benefit from guidance concerning the level of difficulty of books we are using for guided reading. (essentiallearningproducts.com)
Of course, the practical difficulties of getting everyone's DNA on one database, including all visitors to the country, are immense. (newlawjournal.co.uk)
What the term may be referring to is the increasing difficulty of finding suitable book promotion and making the titles stand out. (goodereader.com)