- That’s a surprisingly complicated question. I would generally take book to mean a comic book title. For instance, Batman, Action Comics, and Doomsday Clock would all be comic books, or to be more precise they are titles of ongoing comic book series. I also wouldn’t be surprised if someone referred to a collection of stories as a book though there are other names for those such as a trade paperback or trade hardback collection. These collections of comics are often referred to as volumes. A trade paperback collecting issues 1-6 of Batman would likely be called Batman Volume 1. 7-12 would be Volume 2. Ect. Then adding another wrinkle to this, these volumes usually aren’t the official titles of those collections but subtitles or even just some side of the comic info for people wanting to organize all their volumes more easily. Volumes could also refer to different sets of series with the same name. For instance, the original Batman series launched in 1939 and ran continuously with an ever increasing issue number until 2011 when they relaunched the Batman series with a new #1 issue. The series that launched in 1939 would be Batman Volume 1 and the series that launched in 2011 would be Volume 2. Many sites like MyComicShop refer to comics this way. DC Universe tends to simply refer to titles by their year, so Batman 1939 is the original whereas Batman 2016 would be the latest relaunch in the Rebirth reboot.
Confusing enough for you?
To sum up, you have to know context, but generally I’d assume someone meant the title of a comic series if they said comic book and a collection of comics, either a small group collected in a single book or a name for an entire run of a single series, when they say volume.
- DC essential is simply a list of comic books that DC recommends. It includes some of their most acclaimed titles and important stories for key characters. It’s basically DC putting their best foot forward and saying, "Here’s where we think you should start…though whether those are actually the best place to start is debatable.
Most of the stories you will see in those recommendations will be cannon, but not all of them are. I’m sure The Dark Knight Returns will be in there somewhere, and though this is was a tremendously influential story, it took place in it’s own little universe. Keep in mind that cannon in comics is always kind of in flux. There could easily be a whole two hour lecture to get you the basics on how universe continuity works. I’m sure we can find you some more info on that if you are really curious, but I wouldn’t worry about it too much at this stage. Just enjoy the stories for what they are, little slices into a different more fantastical universe.
- Batman: Year One was a fantastic and influential Batman story in the 80’s, and since then, many characters and teams have had Year One stories that retell and revamp their origins (again, comics canon is constantly in flux). I hadn’t heard of Flash: Year One, but it seems it’s Barry Allen’s chance to get the Year One treatment. As best as I can tell, this is canon tying into the official Rebirth Continuity. If that title interests you, I’d encourage you to start reading from the beginning. I’ve started it myself and read the first arc. It was amazing, and I look forward to reading more. It’s been the same author on Flash for years, and when you get a writer working on the character that long, usually each story ties in with what has come previously kind of like an overarching story in a TV show, so may as well get it from the beginning and stick with it if you like that creative team’s work. Search for Flash 2016 to find the beginning of that series in “Flash Rebirth” and then “Flash #1.”
I’m guessing you are new to comics. If you’ve liked what you’ve tasted so far, you will love DC Universe. If you don’t know where to start, I’d suggest creating thread asking for recommendations. Give people a paragraph describing what characters and/or teams interest you, what you’ve liked that you’ve read so far, what DC movies or cartoons intrigued you, ect. Believe me, you’ll get a ton of directions. Try out different things and see what you like. This world can really grab you.