Anyone know any good books with similar humor to Hitchhikers Guide to the Galaxy?

Red Dwarf is as close as I can think of. In so far as it's set in space and it has that magical breed of British humour.

Well... yes and no.

I've never read anything else that combines humor, wit, philosophy and phrase-turning in quite the same way.

Some that are at least similar in one or another way:

Lots of Terry Pratchett's stuff - I'd especially recommend Guards! Guards! or Monstrous Regiment.

Tom Robbins, and especially Jitterbug Perfume

A Confederacy of Dunces by John Kennedy Toole

Kurt Vonnegut, and especially Cat's Cradle or Sirens of Titan.

Most anything by Carl Hiaasen. He writes in a completely different genre, but with a very similar sense of the absurd.

Will Save the Galaxy for Food by Yahtzee Croshaw.

Rivers of London by Ben Aaronovitch

^ This is a good list. I'll add Christopher Moore's Lamb or A Dirty Job and the comment that imo the Rivers of London series starts a bit skeevy with how women are written, but the author shakes it out by book 2 or 3, and it's got a great voice actor for the audio.
- Yeah - Lamb and A Dirty Job were both pretty good, and I liked Practical Demonkeeping too.
  
  I also thought after I posted that that I should've mentioned Tom Sharpe's Wilt.

Well, there's Dirk Gently's Holistic Detective Agency, The Long Dark Tea-Time of the Soul and The Salmon of Doubt, by the man himself.

Terry Pratchet and his collaboration with Neil Gaimann are worth a look. I'm struggling to recall the title, but there was a book about an interdimensional refrigerator repairman...

Tom Gerencer Intergalactic Refrigerator Repairmen Seldom Carry Cash: And Other Wild Tales

Not exactly the same, but the Discworld books have a great sense of humor and are fun to read.

Edit: didn't notice that autocorrect changed "Discworld" to "Discord."

If you want the same type of humor, Douglas Adams also had the Dirk Gently books.

Terry Pratchett had a similar humor overall, particularly in his discworld series and the book Good Omens that he co-wrote with Neil Gaiman.

But sci-fi wise, I can't think of any.

If you go back to fantasy, you might be okay with Robert Aspirin's myth series. The humor there isn't as close, but it isn't totally different either.

John DeChancie did the Castle Perilous series that, like the Myth series, has a similar enough vibe to the humor that it might work for you

Sometimes the Phase series from Piers Anthony clicks in a similar enough way, but it's a stretch to recommend for this tbh. But Piers Anthony is sometimes a hard read in any of his series, no matter how good they are

But Adams is the only one I know of that did that absurdist, dry British humor in that exact style. People have tried, but failed, because they set out to imitate it, which is a fail from the beginning. And I haven't run across anyone doing it in a sci-fi style that's scratched the same itch at all

Discworld seems to be a very big people are suggesting as well as dirk gently!
- Discworld is a very unique set of works tbh. It ranges from absurdist slapstick with rinsewind, to poignant delicacy in the Tiffany Aching sections, with everything mixed up in between.
  
  It's all set in the same world, but progresses over time and has distinct sub-series with their own tone. But there's Pterry's wit and sense of delight all throughout. I never read the full series until I was an adult, only a book there and there, spaced out over years since the local library didn't have everything. But it still managed to engender a childlike sense of joy and wonder for me. There's not much that can do that at my age lol.

The John dies at the end series might interest you.

One that springs to mind (but the Guide is really really good) is Joseph Heller's Catch-22.

https://en.m.wikipedia.org/wiki/Xanth

Piers Anthony Xanth series is one big pun and joke about Florida.

I haven’t yet seen Heinlein recommended. There is a lot of discussion if Starship Troopers is supposed to be making fun of or supporting the military complex, but I’ve honestly laughed out loud many times reading it.

If you see the humor, I think it’s akin to Adams, but significantly less absurdist, while being way more politically involved. Slightly less political always by Heinlein: Citizen of the Galaxy (on citizenship and societies) and maybe Double Star (on the definition of self). Amazing but not corresponding to what you are looking for Stranger in a Strange Land, about religions, cults and sexuality.

This if you want to stay in sci-fi. The Dirk Gently series was already recommended, if you want to move towards fantasy. This being said, Hitchhiker’s Guide is as famous as it is because it is very unique, so I don’t think there is anything very similar out there. I’d love if you proved me wrong! Let me know!

The Discworld books by Terry Pratchett get compared to Hitchhiker's Guide a lot -- if Hitchhiker's Guide is a sci-fi parody, Discworld is a fantasy parody. HHG I think is a bit zanier/more absurdist but they both are super witty with a side of social commentary.

Mallworld by Somtow Sucharitkul is pretty close as far as being absurdist humor in a sci fi setting. Collection of short stories involving a planet sized mall complete with drugs that make you float and rich people so obsessed with loosing weight they become floating heads.