Since you write that you're a .NET developer and you don't even mention F#, odds are that you're a C# developer. In that case, I'd strongly suggest that you learn F# first. It's another .NET language, and it's a great stepping stone for anyone coming from C#.
In itself, it's a great FP language, but it also offers full interoperability with the existing .NET ecosystem. Besides its FP features, it also offers a full set of OOP features. This enables you to learn gradually.
- Learn the F# syntax, but keep on writing object-oriented code. You can do this in a week or two.
- Start learning functional concepts. For instance, once you understand maps and folds, replace your
for loops with those.
- Learn more abstract functional ways of thinking. Repeat from here.
You can take each of these steps in small increments, all the time staying productive.
Once you have a good grasp of the functional parts of F#, Haskell can be a good next step. I've met many people who tried to learn Haskell 'from scratch'. Most people report that they give up because the learning curve is too steep.
I came to Haskell via some years of F# and found it a natural next step.
There's lots of free learning resources for F#, the most famous of which is probably F# for fun and profit. If you come from C#, another great introduction to F# is Real-World Functional Programming. It juxtaposes examples in C# and F# so that you can compare.
As far as I can tell, there aren't a lot of jobs where you get to program exclusively in F# or Haskell, but learning either will likely still make you a better programmer, so I'd highly recommend it.
I've written F# professionally, but never Haskell. I've written a lot of Haskell semi-professionally; I'm self-employed and invest significant time in doing so, but no-one has ever paid me to program in it. Still, I consider it highly worthwhile.