Reddit reviews Bonewits's Essential Guide to Druidism

I can't speak for all of those books, but you cant go wrong with The Path of Druidry, Druidry Handbook, Druid Magic Handbook, and From the Cauldron Born. I would also recommend:
The Druid Way by Phillip Carr-Gomm
The Mysteeries of Druidry by Brendan Myers
Bonewits's Essential Guide to Druidism by Isaac Bonewits
Principles of Druidry by Emma Restall Orr
The Solitary Druid by Robert Ellison


And please, steer clear of Druid Power and the 21 Lessons of Merlin. If you feel you must, read some of the others ones first before delving into those 2. They rely heavily on fantasy (not mythology) and don't have the good graces to tell you so.

There are not actually very many articles about modern druids, there are organization websites like the one for the Order of Bards, Ovates, and Druids and A Druid Fellowship

And there are several good blogs like John Beckett on Patheos, Damh the Bard (a very well known and influential figure in modern Druidry), and the Archdruid Report by John Michael Greer, the Grand Archdruid of the Ancient Order of Druids in America

Thing to remember is that there are three main approaches to modern Druidry, one is as a religion, people who worship gods, perform rituals, so on, those that approach it as a spirituality and therefor don't hold gods or ritual with as much importance and focus more on themselves than outside forces, and then those who approach it as a philosophy, these people might not even be theistic at all, and tend to focus on the physical world rather than the spiritual one. There are people fall in between of course but that's the basic gist of it. You're going to have to figure out where you fall in that before you have a better idea as to what kind of information and guidance you're looking for.

A few books you could look at are:

The Path of Druidry

The Druidry Handbook

Bonewits's Essential Guide to Druidism

A Brief History of the Druids

It's important to try and read something like that last book especially to give you some sort of context as to how and why modern Druidry came to be. It isn't an ancient religion, we have no ties to the ancient Druids, and very little knowledge about them at all. So to avoid looking foolish, I'd suggest definitely reading something about the history of the Celts and the Druids, and the modern movement.

Also, spiritual ecology books are a good supplement, Druidry is focused on nature so of course learning about nature, ecology, biology, and such is only going to help you. My two favorites are Spiritual Ecology: The Cry of the Earth and Mystery Teachings from the Living Earth: An Introduction to Spiritual Ecology

There's not much unity within paganism. We're a constellation of different faiths, each with their own sometimes very different branches. If you really want to boil it down, then arguably the "Big Three" pagan religions are Wicca, Druidry, and Heathenry, the latter of them being sort of the flagship of the "reconstructionist" movement which is an umbrella of pagan faiths which includes non-Germanic cultures as well.

While I haven't read either of these books myself, I have heard that Bonewits's Essential Guide to Druidism and Drawing Down the Moon are reputable surveys of Druidry and Wicca, respectively, though they are probably a bit dated at this point (especially Drawing Down the Moon). I'm not sure if an equivalent overview of Heathenry has been published.