I have to disagree with KB's comment "It is not difficult to get reasonably good fairly quickly"--guitar has a low "playability" threshold--which is to say that you can be playing at something fairly quickly, but bankable performance skills are as elusive as with other instruments--

You can save a lot of time and trouble if you work with a teacher who is a master in the style of playing that you are interested in,since a teacher will steer you toward what you need, and away from the multitude of distractions and diversions that guitarists can fall into(I think that guitar offers more tangential distractions than any other instrument)--

As a fiddle player, you will have a bit of an adjustment to make, since you will be moving from the foreground to the background, with little positive transfer, since you will be playing everything *but* what you were playing on the fiddle--My experience is that fiddlers feel more affinity to lead guitar than to any of the accompaniment styles--Then there is the whole issue of chords--

I used to teach beginning guitar classes, and never found a book that was very useful--first, because none of them ever tried to find the simplest route from point A to point B, and second, because none of them agreed on where points A and B even were--like most teachers, I used my own fingering and chord charts, and I knew right away where to start with a student, and where, and how fast to go--