
I agree, I can't say that the "sameness" of his lead characters really diminishes the pleasure I get from reading his new books, or even rereading old ones. I'm halfway through reading all of them for a second time, and this feature is apparent but not (yet) disappointing. The story lines are so absorbing that my attention goes mostly there. I do like the idea of a character clearly and significantly different from the type we've come to know and expect.

It will matter a little, but not all that much. Only in a few instances do the main characters carry over from one book to the next. There IS a set of benchmark events that are narrated in some of the early books and are then referenced in later books, so it's fun and interesting to recognize them and then think about how they work. And many secondary characters pop up in various contexts throughout the sequence of books but again, they stand up perfectly well on their own in each book.
I read them in order because I was interested in seeing how Furst's narrative technique evolved, and then when I came to understand how he used these background events and characters to connect what are basically standalone novels, it just added that much more to my reading pleasure.
Since your friend has read books 3 and 4 and is presumably a fan, I probably would go back to Night Soldiers (1) and/or Dark Star (2) to fill in some of the missing pieces. Especially fun is a scene about an assassination in a Paris restaurant, the Brasserie Heininger, from Night Soldiers that shows in every single subsequent story.

I think I like and remember more the early leads, as they seem more developed. I am reading my way through the whole series for the second time and ration them out from month to month, too, because they're such fun and go so fast.

I haven't read anything before Night Soldiers. There are interesting comments about them under the book links themselves, but if anyone has opinions, it'd be fun to hear them.

Furst's stories have clearly evolved. The earlier books tend to show longer stretches of characters' lives, which establishes strong emotional ties to them. But the stories are kind of shaggy dogs. The energy ebbs and flows. The later books have tighter storylines. We see narrower slices of characters, though, with less back-story and development. And the lead characters tend to start seeming fairly similar--early- to mid-40's, fairly solitary, hard for interesting women to resist, but still profiled as "common" guys swept up by the tides of history.

I think Nikko Stoianev from Furst's first book, Night Soldiers, is interesting, because we see him through so much of his life. My brother likes Jean Casson from The World at Night and Red Gold.