Welcome Johnh,
looking around you'll find as many suggestions about where to go next, that you'll be disoriented. A lot. (There's even a section here named 'So many books...where the hell do I start?')
But, I will give you the way to go now! - for free!
- Buks poetry is kind of his Main work - but NOTHING for beginners (except you're into poetry anyway)
- His short-stories vary a LOT in quality (due to the fact that some were just written to make money with girlie-mags.)
- You did like a novel of his - so for a start: stick to this!
I see 3 possible novels for you to enter next. Depending on What you liked most with 'Post-Office':
- the restless boozer who goes from one job to another:
That's 'FACTOTUM'
- the womanizer, who made his success (this one is settled right after 'PO') through writing. But still not satisfied, still looking for something really fulfilling.
That's 'WOMEN'
- the kid (childhood and adolencense) - we see, where all this came from: the loner, the drinker, the writer/poet, the outsider, the tortured creature.
A beating father, schoolmates that hated his guts, acne, no chances with the girls, anti-germanism, the big depression, no hope, no future, a lost soul.
This is 'HAM ON RYE'.