Assuming that Mendelian genetics were known in his time he would have been able to write a sounder book; the fact that like everyone else he was hazy on inheritance of traits is a shortcoming in hindsight. It would certainly not have led him away from evolution.
Since most of Mendel's studies were between 1856 to 1863 (although he had a couple of prior papers published around 1854 and Darwin's "Origin of Species" was published in 1859), I would doubt that Darwin had much knowledge, if any, of Mendel's work on plant hybridization.
I think it would have had an effect on some of the mechanisms that Darwin proposed for Evolution to be sure. For example, the "survival of the fittest" theory would deserve another look as no one organism hands down 100% of it's genetics to it's offspring even if it breeds within it's own species (the genetics of the offspring would depend on the chosen mates genepool, the probability of a certain mixture coming out for any one child, and many other factors beyond an organisms "fitness" all introduced from Mendel forward).
In other words, thanks to Mendel we know that a very successful organism can be the carrier to many flawed genes that can lead to less than successful offspring (whereas in Darwin's explanation success in one generation directly leads to survival in the next if the environment remains mostly static...which if it were true would mean that most of the diversity on earth would have long since become extinct...).