As an alternative, I might suggest starting with "Guards! Guards!" since it's one of the good plot arc beginnings and actually directly involves the library and L-Space.
I'm also a bit less fond of "The Colour of Magic" since I think his writing style was weaker in his first few books and since his characters were not well established. Both "The Colour of Magic" and "The Light Fanstastic" are very fun books but not quite at the same quality level as his later ones once he found his rhythm and built up his world.
I'm also a bit less fond of "The Colour of Magic" since I think his writing style was weaker in his first few books and since his characters were not well established. Both "The Colour of Magic" and "The Light Fanstastic" are very fun books but not quite at the same quality level as his later ones once he found his rhythm and built up his world.