If it varies quite a bit from book to book then I wonder if it is up to the publisher to decide what is included in the sample? It seems to me that two things are very helpful in choosing a book based on a sample thereof: one, does it have a decent TOC (though this is probably more important for non-fiction), and two, how well written is the first chapter.

I've been very interested in books after reading their supposed content description but then looked at the TOC in the sample and realized the content of the book was not what I initially thought it would be. The total lack of a TOC is often a turn off too as it makes navigation a major pain.

Perhaps if they required a sample to contain the TOC and at least ten pages of the first chapter one could better decide whether to purchase or not.

Maybe they should look at how Amazon does samples as I've never encountered one of their samples that didn't have at least several pages of text to preview.