Is there any way to add it into the sight if you already own a book, be it in physical or PDF form?  I'd rather not have to purchase a book twice if it isn't necessary.  Also is there any plans to add any 4th or 3rd edition books on here?  Sorry, I'm still kinda new to the site.

D&D Beyond vs Physical Books: An Explanation

Basically: the answer is no. You did not purchase the books from D&D Beyond so if you want to use them on D&D Beyond you will need to purchase them again. However, you can purchase things piecemeal: only need a specific class or spell etc? Then buy only that class or spell. Or buy the class/spell/etc bundles. If you later decide to buy the full book any piecemeal purchase made will discount the book by the amount you paid. Any such purchases will also contribute as a discount towards a Legendary Bundle.

Due to licensing, time, cost, etc previous books will not be available on this site.

PDFs will not be available and this was the decision by Wizards of the Coast, the publishers, and D&D Beyond have no control over this.

No, it is not possible to unlock content here for free if you own the physical books.  It's is extremely unlikely it ever will be.  The reasons for this have been discussed to death in other threads.  I hope by PDFs you are talking about things like the Tortle package, as there are no legal pdfs of the hardcover books.

What you can do, if you have the time and inclination, is create private homebrew here that replicates the character options, monsters, and magic items from the books.  You just can't publish them here for others to use.  Or, if you want to use the site primarily as a character builder, and you know just what you want to build, you can just buy the individual items (race, subclass, etc.) you need to build that character.  Those purchases apply toward the purchase of the entire "book" from which they came, so you can pace your purchases and eventually end up with the whole "book" or "bundle" of options.  If this is something you'd like to consider, you might find the buyer's guide in my signature helpful.

There are no plans to support 3e or 4e.  The DDB team still has a tremendous number of features they want to add for 5e, and have their hands full with that.

Okay, thanks for the info.  Didn't realize you could buy things piecemeal.  Also, a bit perturbed I didn't get notifications I had gotten an answer to the thread, but I at least am getting the info.  I mainly just need certain classes and races which I have a habit of playing.  Mages, bloodhunters, tieflings, rogues and the like.  My info on the bloodhunter is probably out of date anyway now that the book on Critical Role's game world is out and I am sure it is in there now and tweaked as well.  I do wish they'd like add a thing where when you buy a physical book you get a way to also get it in D&D Beyond though.  Would be super nice.

Do you have links to any of these threads that I might read the reasons?

Essentially it boils down to this, Zanfeth. The DDB website is hosted, owned, and run by Curse (soon to be a merger with Fandom apparently) as is not a subsidiary of WotC and have no direct involvement with WotC other than an agreement that they can provide this service for monetary gain. Therefore, DDB gets absolutely no profit from any books purchased either directly from WotC or a third party distributor and thus they make their profits by selling this service through their agreement with WotC. There is also no way for Curse to verify your purchase of any physical books as, again, they are not a part of WotC. I hope that clears things up for you.

Quote from Lyrel >>

Essentially it boils down to this, Zanfeth. The DDB website is hosted, owned, and run by Curse (soon to be a merger with Fandom apparently) as is not a subsidiary of WotC and have no direct involvement with WotC other than an agreement that they can provide this service for monetary gain. Therefore, DDB gets absolutely no profit from any books purchased either directly from WotC or a third party distributor and thus they make their profits by selling this service through their agreement with WotC. There is also no way for Curse to verify your purchase of any physical books as, again, they are not a part of WotC. I hope that clears things up for you.

 Just a tiny correction: the impossibility of validating any physical book purchase does not come so much from Curse not being WotC, as to there not being any unique code associated with each copy of a certain book. This is indeed due to WotC seemingly not be willing to include such codes in their products, but should they be introduced in the future Curse could make use of these unique codes even without being effectively part of WotC.

Quote from Zanfeth >>

Okay, thanks for the info.  Didn't realize you could buy things piecemeal.  Also, a bit perturbed I didn't get notifications I had gotten an answer to the thread, but I at least am getting the info.  I mainly just need certain classes and races which I have a habit of playing.  Mages, bloodhunters, tieflings, rogues and the like.  My info on the bloodhunter is probably out of date anyway now that the book on Critical Role's game world is out and I am sure it is in there now and tweaked as well.  I do wish they'd like add a thing where when you buy a physical book you get a way to also get it in D&D Beyond though.  Would be super nice.

 
The Blood Hunter class created by Matthew Mercer from Critical Role was not featured in his Tal'Dorei Campaign Guide due to it already being released on DMs Guild, the same going for the Gunslinger Fighter Archetype he created. However, D&D Beyond does feature both the Gunslinger and the Blood Hunter. As far as I know they are kept up to date with latest revisions and tweaks. The campaign guide is not yet available - there is difficulty in getting third party content released on the site and the campaign guide was released under a completely different publisher, Green Ronin Publishing. For it to be available here D&D Beyond, WotC, Matt Mercer, and Green Ronin Publishing all have to agree on a deal. It's complicated.

But, as I said, that class and subclass are available here. When making a new character on the home page of the builder there is a toggle switch for Critical Role, if you turn this on the Blood Hunter class becomes available and so does the Gunslinger (which can be selected for a Level 3 fighter). If you also turn on the Playtest toggle you can also select the Firearm Specialist feat Matt created.

Link to Blood Hunter class: https://www.dndbeyond.com/characters/classes/blood-hunter
Link to Gunslinger Archetype: https://www.dndbeyond.com/characters/classes/gunslinger

Also note if you have the book physically and there is a subclass, feat, race, monster, magic item, or background you want then you can create it as Homebrew on the site for private use. See the Homebrew forum for pinned guides.

Quote from Zanfeth >>

Do you have links to any of these threads that I might read the reasons?

Cyb3rM1nd gave you one link in post #2.  There's another long thread somewhere in the forums; it used to be pinned, but appears to no longer be pinned, and I haven't been able to find it.

There is also this video:  https://youtu.be/bu9ZIFjJIX4  My quibble with the video, as helpful as it is in adressing some issues, is that while Nike shoes are the same whether purchased from Nike or a distributor, what you get from purchasing on DDB and from purchasing a physical DnD book are not the same.  yes, all the text and artwork of the books is reproduced here, but there is also so much more:  tooltipping of monsters, spells, items, conditions, etc.  Links between various parts of the books.  All the tools (monster listings, spell listings, character builder, etc.).  All that takes time and effort from developers who deserve to be paid for their work.