How hard is it to learn?

I'm wondering because I have a couple stacks of crime/detective novels that have been translated into English, and I'm curious as to what they're like in their native language. Also wondering what good books I'm missing because they haven't been translated.

I want to lean so I can add more books to my already overloaded shelves.

For a native English speaker? Trickier than picking up one of the Romantics (Italian, French, Spanish, Portuguese) not as hard as learning one of the Asian languages.

Basically with English you originally had a form of Dutch that mingled with Gaelic (which is where that crazy -ing gerrund comes from). Then the Vikings invaded and you had an influence from Norse (I believe that is when English lost its genders) and then the Normans invaded and you suddenly had a huge shift towards French. And then if all that wasn't messed up enough it was arbitrarily decided English should follow Latin rules, but only some of them (I think that was the Church's fault during the Renaissance).

So because English shares vocabulary with the Romance languages and some of the rules, Romance languages are generally the easiest to pick up.

Because English used to be based in Dutch, Germanic languages are still totally doable, just a bit harder because you have to throw out all the Latin nonsense.

There's basically almost nothing left from the Gaelic, so that's really hard (plus Gaelic is just crazy, or at least that's my opinion).

And then things like the Asian languages, African languages, Native American languages that share no common ground at all with English are the hardest for a native English speaker to learn.
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Just don't try to learn Danish. Seriously. You'll never get the pronunciation right. Swedish is way easier than Danish.

However, if you're never going to communicate orally in Danish, it won't be that difficult. Same goes for Norwegian. Swedes, Norwegians and Danes understand each other to some extent: if it weren't for national borders, the three would very likely be considered dialects of the Scandinavian language. Much like different dialects of Chinese would be considered separate languages, in fact.

Anyway, Swedish (or Scandinavian, if you like) supposedly isn't terribly difficult. That's what I've been told, anyway. It's certainly easier than Finnish, but that's nothing to go by since Finnish is generally considered one of the most difficult languages in the world to pick up (and it shared very few characteristics with the Scandinavian language(s), so there you go).

Finnish seems pretty easy to learn to me. :P

I am a native English speaker, but I do know basic French, Spanish and German. Better at understanding spoken and written rather than speaking them myself.

I am looking to understand written and spoken Swedish, maybe write it myself. Not sure if I'd ever get a chance to try and speak it myself (tho I would love to travel there someday).

Where would a good place to start be?

There is a lot of old norse words in English language, so a surprising amount of words are similar. Where the largest differences lie is in grammar and syntax, for example with the gender specific articles.

Swedish films, television and books can be a good start. Astrid Lingren's books are quite good starting points. She wrote children's books, but they are very good stories. Check out "The Brothers Lionheart" or "Mio, my son" as well as her more famous series. I would also recommend the Hamilton series by Jan Guillou when you are ready to move on to more adult language.

Sweden have a nice output of films, quite a lot of them with English subtitles, also a good starting point to listen to how the language works.