I asked the same question once, so many back then had large paperback books, MASH, Law and Order, All in the Family, many 70's shows that were on over 6 years. For whatever reason, no one wanted to do it, I'm sure to be "official" it had to have the producers approval.
They weren't into many products either, lunchboxes, some games, but nothing like a LHOP cabin, horse/buggy, store, things that kids would like.
Some people have written about the show,)(not cast members but their books were sold at events, not amazon or online as far as I know. If I see any, I'll post it.