A rule of thumb with James is that, the later the novel, the more psychological, abstract, and long the prose. For ease of reading, go for earlier works like Daisy Miller. On the other hand, for full-bore Jamesian brilliance, read later works like The Golden Bowl. Bear in mind that James revised his corpus for the New York Edition around 1908, and in many cases the changes were substantial. The Norton Critical Edition of The Portrait of a Lady, which is early, for instance, uses the 1908 edition. So in that edition you have early James as revised by late James.

It is unfortunate that the default unit of this site is the "book," a term meaningless in some periods (think of "Beowulf") and unhelpful in the case of writers of short stories. Melville's most famous short stories are "Bartleby" (also published as "Bartleby, the Scrivener") and "Benito Cereno," both of which I highly recommend. I honestly think it's pointless to embark on Melville with anything less than "Moby Dick," but if you feel you can't handle 500 pages of seafaring goodness quite yet, start with the slim "Billy Budd" (published in the 1920s, but written, obviously, during Melville's lifetime).

When you feel like a Melville pro, go for "Pierre," his "Kraken" (the kraken is a sea monster said to be bigger than a whale; this is how Melville characterized "Pierre" in relation to "Moby Dick." Do NOT, however, read the "Kraken Edition" (Harper), no matter how delightful you find the illustrations by Maurice Sendak (and they are delightfully grotesque). The editor of that edition, Herschel Parker, went through and eliminated all references to Pierre as a writer, on the theory that these were all a result of Melville's frustration with his publisher and not "originally" part of the novel. The critical consensus is that Parker's a nut. Read the Newberry Library-Northwestern U edition, which a younger and more sane Parker helped edit.