If you speak English most people in the Netherlands probably already speak it better than you.

I used to work for a publisher in the Netherlands called Coutinho, they specialize in second language acquisition and have some excellent materials on Dutch. Probably not for free, though.

I've started using Dutch in 3 Months, by Hugo. Ignoring the timeframe the title gives, it seems decent because it has an app alongside it for hearing the pronunciations of each word, so you learn to speak and read essentially at the same time.

I got my book online.

In my own personal experience: live in The Netherlands and go work in a small company were pretty much everybody is Dutch and the management wants to help you learn Dutch (so they keep doing all meetings in Dutch even though everybody there could speak English).

After a couple of years living there working in English-speaking multinational environments and thus without learning much Dutch, that specific situation forced my Dutch speaking skills to improve massively in a mere 3 months.

Otherwise it's pretty hard to learn Dutch, IMHO, unless you're starting from German (even better, Swiss-German) which is maybe the closest language to it (and Dutch grammar is actually simpler).

For spoken language learning, nothing beats the Pimsleur method. Easily found by those, who sail the high seas