Woodrow Wilson Smith, aka Lazarus Long, was the protagonist of the 1958 novel Methuselah's Children, which was set in Heinlein's "Future History" universe. This novel bridges that universe into his later novels by focusing on the efforts to convince a superannuated Lazarus to accept rejuvenation treatments.
Its highlights are some of my favorite bits of Heinleinia: a look at pioneering on a low tech colony and time travel to pre-World War One America. It's marred by Heinlein's habit of nerding out in multipage deep dives into things like orbital mechanics or (in this case) genetics. It's also marred by the protagonist going all oedipal on his mom in 1917; just when you want the viewpoint character to continue looking around the early 20th Century U.S.A., he gets scope-locked by a crush on his own mother.
All in all, it's worth the read, but there are definitely parts you can skip on a re-read.
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