what book is this?
 

There's a planet, and every so often another planet come close to the first one. And threads start falling from the sky. On these threads ride dragons and they come to invade the first planet.

http://kumo.swcp.com/~quirk/afp-index.shtml
 

Although the plot summary is a little off, most certainly you are talking about Anne McCaffrey's The Dragonriders of Pern ® books.

I read a few as a teenager and, although engaging reads, they were a little too saccharine for my taste: I did not continue with the series that now extends to a dozen or more volumes.

hey thanks, Benedict

would you know in which order I should read the books?

At the following site there are links to a chronology and a suggested reading order: http://pern.srellim.org/bookinfo.htm

For some Pern appreciation from other BookTalkers, see this earlier thread which also included a suggested reading order which may or may not accord with the one at the above link: http://www.dvdtalk.com/forum/showthread.php?t=122878

okay, thanks!