I've had good results with Calibre - that seems to be the one most people recommend, and it's multi-platform, which is a bonus. There are also online 'convert anything to anything' services. I think Zamzar is the most popular one, but a dedicated app is probably the best way to go.
Good to hear you're all sorted on the proof-reading front. Thanks Codetapper. Yes, it is quite jarring to read - my stupid brain keeps correcting and tweaking everything on autopilot rather than just absorbing and enjoying the content. Otherwise it would flow nicely, and I could pretend I'm reading Amiga Power back in '91.
Keep in mind there are quite a lot of instances where a spell-checker won't help e.g. choosing when to use 'to' or 'too', 'of' or 'off', correctly spelt words, but in the wrong tense or context. I'd also avoid using the same word two or more times in the same sentence. It's not exactly bad grammar, just not good form.
The content remains top-notch, which is the important thing.