In chapters 16-18 what does Krakauer discuss that he didn't discuss in previous chapters? And why did he delay presenting this information?

The final section of Into the Wild is especially tragic, in that it shows that McCandless, at least from what little evidence is available from his last weeks, had matured, and was ready to rejoin society. I don't believe this was discussed in previous chapters. I think he wanted the reader to interpret this before he came out and stated it.