I didn't make a specific recommendation since the original books I bought are a few years old (we're talking Photoshop CS2). At the time I got
this one (other vendors are available). If you search for `nature photography photoshop' you'll find something more up to date, and I see that at least some of these books get good reviews on Amazon.
In terms of workflow I moved over to Lightroom some years ago. Two of the main authors have written on that as well as on Photoshop. All I can comment on is style, which is a bit of a personal preference. Scott Kelby's books are chatty, with a lot of informal language, but he does have a technique he's trying to teach in each section and I had no problems with understanding the ones I looked at in detail. Martin Evening's book have a more serious tone, and they're more methodical. In his books there will be some fairly dry material before you get started, but he does prompt you to think about issues you ought to think about at some point, so that you don't have to change what you do later on. It takes longer in his books to get to the practical side, and there are fewer `here's how to solve reasonably common problem X' bits in them.
I haven't tried any of the other authors. One of the very first books I ever read was about digital workflow, and I'd really highly recommend getting one of those, or picking up one on Photoshop which talks about that in some detail.
These books aren't cheap, but a lot of them are available on the used market for signficiant discounts. I do think it's worth investing time at the beginning in working out what the standard options are, what is and what isn't important for you, and then to develop an order of doing things that makes sense to you and that works for your images and your objectives. Once you have all those, it becomes much easier to develop that (because some things become more important, or because the software develops), and to use tutorials for dealing with specific issues.
Andrea