What’s the meaning of the underlined spin-off titles and more?

The passage below is from The Library: A Fragile History by Andrew Pettegree.

Public libraries, it was clear, had survived their encounter with competing media, which by the 1960s also included television, seen by many commentators as yet another existential threat. For television offered consumers the best of both worlds: enjoyed, like radio, in the home, with the potent visual

appeal of cinema. But again, as with radio, the book world would profit from the obvious crossover opportunities afforded by new editions of books dramatised on television, epitomised by the unlikely success of John Galsworthy’s Forsyte Saga (1906–21), a set of which would decorate the shelves of millions of homes after a twenty-six-part serialisation by the BBC in 1967. Documentaries and comedy series proved equally lucrative sources of spin-off titles, which boosted the revenues of the BBC and increasingly dominated the UK bestselling lists by the 1980s.

In this passage the underlined parts trouble me.

John Galsworthy’s Forsyte Saga was a set of books that dramatised in 1967.

Then does ‘spin-off titles’ mean books that were adapted to the use of documentaries and comedy series in this context?

Am I right?

Last but not less easy, what does the underlined which exactly refer to?

Does it stand for ‘spin-off titles’ or the whole part before ‘which’ or ‘Documentaries and comedy series’. I cannot decide which.

Thanks in advance.