Looking for recommendations for good time travel or parallel universe or alternate reality novels.

Think "Sliders" etc

Definitely no young adult (unless the insta-luv is non-existant or very light)

I like a bit of light hearted humour also, but doesn't necessarily have to have that.

edit: or short story/anthologies

[Lizand]

Parallel universe: the trilogy by Robert Sawyer that starts with Hominids, then Humans, and I've forgotten the last title.

Time travel:
Connie Willis makes this a specialty. My favorite of hers is The Doomsday book. To Say Nothing of the Dog is a lighter novel. Passages is pretty good too, although technically it might not be time travel. She has a2-book set on the London Blitz.
Michael Crichton's Timeline.
Daphne DuMaurier's The House On The Strand.
Audrey Niffenegger's The Time Travelers Wife.
Stephen King's 11/28/63 -- I think I'm wrong on the 28...

Not exactly time travel but controlled reincarnation:
Life After Life by Kate Atkinson.
The Many Lives of Harry August.

I've read Audrey Niffenegger's The Time Travelers Wife, i think you'd like it

H. Beam Piper's - Paratime series.
Gregory Benford - Timescape.
Michael Moorcock - Behold the Man.
Not a big fan of the following author but, Harry Turtledove

I LOVE The Doomsday Book - one of my favourites I've read a few of Connie Willis and the blitz ones I like...the titanic one I hated. LOL.

The only ones I haven't read in your recs are the Daphne DuMaurier one and the Robert Sawyer ones. Thank you for that - I'll look them up!

Parallel universe: the trilogy by Robert Sawyer that starts with Hominids, then Humans, and I've forgotten the last title.

Time travel:
Connie Willis makes this a specialty. My favorite of hers is The Doomsday book. To Say Nothing of the Dog is a lighter novel. Passages is pretty good too, although technically it might not be time travel. She has a2-book set on the London Blitz.
Michael Crichton's Timeline.
Daphne DuMaurier's The House On The Strand.
Audrey Niffenegger's The Time Travelers Wife.
Stephen King's 11/28/63 -- I think I'm wrong on the 28...

Not exactly time travel but controlled reincarnation:
Life After Life by Kate Atkinson.
The Many Lives of Harry August.

The Outlander Series by Diana Gabaldon is my favourite time travel series. Currently reading Time And Time Again by Ben Elton and enjoying it so far.

[webhill]

This is like the fifth time in a week someone has recommended Diana Gabaldon's Outlander Series to me. This must mean something... LOL.

Try Gareth P. Jones' No True Echo. The echo technology in the story enables time-hopping. It's really cool

The riftwar series by raymond feist is the best parallel universe series that has time travel in some parts. Great series that even goes farther into the parallel universes with other series feist has written that are from the point of view from the other world.

[firewing]

If you haven't read Stephen King's The Tower/Gunslinger series, try it out for parallel universe. Also, a series by Mark Anthony called The Last Rune, which is medieval/modern-day Colorado [6 books].

Not exactly time travel but a time loop a la Groundhog Day, in a fantasy setting : Mother of Learning. A free and still ongoing webfiction, probably one of the best and quite famous but you may not know it. Some of it is published and is available via amazon.

You could also check The Anubis Gates by Tim Powers. One of my favorites. The premise is based on time travel.

The first fifteen lives of Harry August, by Claire North is one of the very best time-travelling books I've read. I really can't recommend it enough, do check it out.

[bookworm28]

For parallel universe try Terry Pratchett and Steven Baxter's The Long Earth set of books. They're very good. Reminded me a bit of Sliders for some reason.

Smartbitches recommended Passenger by NOT ALLOWED, and I gave it a try, and recommend it strongly.

It's the first time-traveling book I've read where the big and small prejudices of everyday life are mentioned - a woman's difficulty to move around on her own, for example, or the weight of dresses, or the uncomfort of stays. The hero is biracial, during the American Civil War, so there are a whole other set of problems, which are addressed

It IS a YA, but no insta-love as you mentioned, and no teen angst.

Time Salvager by Wesley Chu