I haven't read a lot of Leigh Brackett - an adventure novel or two. I think of her as a solid, but uninspiring pulp author. The Long Tomorrow changed some aspects of my view.
The writing is stronger than I anticipated/remembered, and Brackett does a nice job of concisely conveying an interesting post-apocalyptic world, and of creating credible, engaging characters to follow. She does a great job of showing us Len's moral and emotional struggles, with turning him into a caricature. While Len is the core character, Esau gets fair treatment as a secondary character, and the two are interesting throughout. There's a fairly strong Tom Sawyer feel throughout, though of course the plots are entirely different.
The book was written well over half a century ago, but stands up remarkably well, in most ways. While the concept is by now a very familiar one,the focus here is on Len and his maturation. Where Brackett fails substantially (though slightly mitigated by the book's age) is in the treatment of women. While there are important female characters, they're entirely designed as plot mechanisms, rather than real people. Both of the boys' love interests are manipulative and needy. It makes a little sense in the male-dominated world of the book, but it's still unpleasant to read. Pretty much the only positive female characters occur early in the book, in the form of standard mother and grandmother types. Given the quality of the rest of the writing, it gives the feel of a writer trapped in the pulp conventions of the time, or unwilling to fight against them.
Despite its flaws, this is an interesting book deserving of a read.