I have just ordered several Japanese books which I plan to go through, along with DOIJG and DOAJG, over this coming year. Something I'm wondering about though - is it better to have a big anki deck where I put lots of sentences from different books in, or should I try to have many anki decks, one for each different book I use? As far as I can tell it wouldn't actually make any difference, but if someone could share any experiences/tips, I'd be greatful. Thanks.

I recommend just one having one, I used to have diff one's but it was a bit of a pain remembering to do them all. Also it's harder to keep track of how much reviews you have going on.

I can't think of any disadvantages to 1 big deck.

I'd make one deck, just for simplicity. If you'd like to keep track of where you got the sentences, you could always add a "source" field to the card model, or use tags. Consider whether you'd actually use it or not, though. The less deck maintenance, the better.

I use separate decks for separate types of studies. For example, all vocab goes into one big vocab deck, regardless of where it comes from. All grammar sentences go into my grammar deck. All output sentences go into my output deck.

I like splitting different types of study up, however, regardless of source, if it's the same type it should go into one deck. Otherwise things wind up cluttered.

Well, that's what I like to do, as well. I just inferred OP was making a sentence deck. I agree with splitting things up based on study type, but not by sources. The latter feels like it would turn out very cluttered indeed, especially as additional sources are acquired.

I used to use multiple decks, but then I merged them. Since I have done that my recall has much improved since it is no longer easier to guess based on the deck it places more emphasis on understanding I feel.

I now use two decks a "silent deck" for work and a "media" deck for when at home.

I have two max . One is my own created deck and the other one is premade so I can add more when I don't feel like adding to my original deck

I have tried various ways of splitting my learning between several decks, but I always end up neglecting one or more of them. So I now work with just one large deck. This has certainly been much more productive for me.

I'm always merging/splitting/evolving my decks.

I've found that different types of study call for different decks. If I'm doing my RtK reviews, I don't really want grammar being thrown at me...I'm doing RtK.

One downside might be that "but you should be able to recognize it regardless of context!"
I'm not saying that I don't recognize it -- it just changes the pace, and it annoys me.

I'd suggest playing around with it and see what works for you. I like separate decks because it keeps things consistent.

I got a couple decks, RTK Part 1 deck that I do less often. RTK Part 2&3, which I try to do daily. This lets me focus more on things I've recently learned, instead of my most mature cards that I don't have any trouble remembering even if they are 2 months expired. I probably should merge RTK Part 1&2, but I've been lazy. And I have two Core2K decks, a main one, and a disposable one strictly for learning which is at a cramming type interval.

Realistically, I only really use two of my decks frequently.

I use tags to keep track of where I originally get the material for my cards. That way I can easily retire (or temporarily retire) cards that are old or from source material I don't care about anymore.

I agree with the general sentiment that it only makes sense to have separate decks when their content is mutually incongruous.

I have two decks:

Recognition Deck (i.e. vocab and grammar deck)
Unless you use clozed delete cards, I don't really see a point in separating vocab and grammar into different decks. Essentially, vocab and grammar cards are reading cards. And since they're reading cards, I don't lose my pace when I switch between them. Hence why I like to keep them in a single deck.

Production Deck (i.e. kanji deck)
I keep kanji cards in a separate deck because I lose my pace when studying them with other types of cards. The first reason for this is that writing kanji is production whereas reading is recognition. The second reason is that I always use a pen and paper when reviewing kanji. Thus, I'd have to keep moving back and forth from my keyboard to my notebook if I combined my kanji and reading cards. Clearly, this would be a waste time.

On a separate but related note, I do most of my reviews on my iPhone while I'm on-the-go. I can't just pull out a notebook and pen to write some kanji when I'm standing in line at the grocery store. This is another reason why it's good to separate reading and kanji cards into different decks.

When decks grow larger, I do notice that the startup loading speed drops. Futhermore, if a deck goes corrupt, with a larger deck you lose more. But those are only minor disadvantages. Frequent backups and patience mitigate them.