Angus Dunnington's Winning with Unorthodox Openings [Everyman] gives on page 11 the line 1.b4 e5 2. Bb2 f6 3. b5 d5 4.e3 with the comment:
"Black must decide what his ambitions are in the centre"...and gives 4...c5 and 4...Be6 as Black's main choices.
Then on page 23 the line 1.b4 d5 2. Bb2 f6 3. e3 e5 is given, where our guide remarks:
"4. b5 a6 ... leads to helpful simplifications for Black ... with equal chances"So presumably that's Black's best idea in the other move order, and maybe White should hold back from 3. b5.
It's a bugger spotting transpositions I know; less forgivable is omitting a whole main line:
Angus Dunnington's Winning with Unorthodox Openings [Everyman] gives on page 91 the line 1.f4 e5 2.fxe5 d6 3.exd6 Bxd6 4.Nf3 Nf6 5.g3 h5 with a fine game by Tartakower.
If you ever get this line as Black against Dunnington, I would be interested to see how he replies to 5...Nc6, 5...Ng4 or indeed any of the common replies to 5.g3 other than 5...h5 that are on my database, and completely unmentioned in the book.
[Sorry to pick on Angus again, I guess it shows I'm sufficiently interested in his books to by and study them. (Although Chess Psychology was an absolute dog, winning a prize only for the most uses of the word "ostensibly" in a chess book.)]
What's the problem here? I think it's repertoire books being written by people who don't actually specialise in (and may never have touched) the openings in question. It also suggests a test: how many games cited in the book are by the author? If you skim a chapter and turn up none at all, add a pinch of salt, or a bit of additional study, to your reading.