MrsD,
Asimov wrote the best, most complex, complete Time Travel novel in The End of Eternity IMO. I have read all his works a multitude of times.
Another favourite author - when my eyes allowed - was Terry Pratchett, the superb Discworld series. Each book stands alone, but if you like strong female leads in well-crafted, humerous Fantasy I would suggest The Witches Trilogy: Equal Rites, Wyrd Sisters and Witches Abroad to start. Then start at the start of the series with The Colour of Magic - a Wizards novel. Pratchett took some time to develop his storytelling style, so it is best to start when he has found it as you are then sympathetic to the Characters in the earlier books despite a few writing problems.
On the whole, the Discworld series takes you on a wonderous journey right up until Terry succumbed to Alzheimer's.

Dave