I once had a theory that the books were actually reports written post-war, possibly something they were forced to do to account for their actions during the war years. Sadly there are too many flaws in this theory. Rachel's POV would be impossible to write post-war of course, plus there are things in the reports that I'm sure the Animorphs wouldn't willingly divulge, such as the existence of the Chee and Ellimist/Crayak.
I suppose their thoughts and memories could have been downloaded using technology similar to that used by the Iskoort in #26 to record and purchase memories. Everything before #26 was already recorded by the Iskoort anyway, and it's possible the Chee could have recreated a similar technology to use later. Assuming said technology still works on dead people, they could have extracted Rachel's memories when her body was recovered.