Cryonicists and other immortalists love "Methuselah's Children," for obvious reasons. The males among us would like to be model ourselves on Lazarus Long, at least in his earlier stages. Unfortunately Heinlein's imagination faltered in "Time Enough for Love" and later novels, where he has Lazarus living a succession of recognizably human lives, over and over again across the centuries, instead of reaching for something that an ordinary, short-lived human wouldn't have contemplated doing or thinking or becoming.
Nonetheless, Heinlein's novels can supply the Transhumanist with spiritual fuel, provided that you keep him in perspective. Heinlein wound up writing science fiction almost by default because he was an invalid, and thus had to transform his desire for a more active life into his fantasies of superior health and longevity, space travel, interstellar wars, political revolutions and polymorphous-perverse sex. His fantasies about the options available to the superior man or woman have to be taken with a grain of salt, much like the similar fantasies of the agoraphobic Ayn Rand. Although novels are as legitimate a source of ideas for practical living as anything else, the young, impressionable youngsters (and the chronologically older individuals still stuck with an adolescent psychology) who think Heinlein could provide a guide to real life need to be careful.