The Time Traders, by Andre Norton

Andre Norton

Andre Alice Norton, née Alice Mary Norton was an American science fiction and fantasy author under the noms de plume Andre Norton, Andrew North and Allen Weston...

, is the first volume of a series of related time- and space-travel stories and part of Norton's "Forerunner" universe. Volumes in this sequence are:

• The Time Traders (1958)
• Galactic Derelict (1959)
• The Defiant Agents (1962)
• Key Out of Time (1963)
• Firehand (1994) (written with Pauline M Griffin)
• Echoes in Time (1999) (with Sherwood Smith
  Sherwood Smith
  Sherwood Smith writes fantasy and science fiction for young adult as well as adults. She has participated in and organized writing groups for many years.Smith's works include the YA novel Crown Duel...
  
  )
• Atlantis Endgame (2002) (with Sherwood Smith)

Sherwood Smith's collaborative volumes are more polished than Norton's solo work, while retaining the feel of the classic entries in the series.

Project Gutenberg

Project Gutenberg

Project Gutenberg is a volunteer effort to digitize and archive cultural works, to "encourage the creation and distribution of eBooks". Founded in 1971 by Michael S. Hart, it is the oldest digital library. Most of the items in its collection are the full texts of public domain books...

 has some examples from this series:

• The Time Traders
• Key Out of Time

LibriVox

LibriVox

LibriVox is an online digital library of free public domain audiobooks, read by volunteers and is probably, since 2007, the world's most prolific audiobook publisher...

has an audio book version:

• The Time Traders