My husband's a chef and his mother was a food scholar, and we have 100s, if not 1,000s of cookbooks. Most of which he never uses. I don't like books stored two deep, but if you don't really need access to them, I don't see why deep shelves wouldn't work.

Personally, I'd rather have the closet than a bookshelf. I have too much stuff that needs to be contained behind closed doors.

It sounds like your build will be custom, so I suggest building the shelves as very shallow drawers which can be fully extended to access the back row of cook books. Possibly have the "back row" set on an incline so that they tilt making the spines visable to see each edition.

No wasted space, everything accessible and no worries if you purge because then the front row will still work alone.

Yes, if you can't pull the shelves out to get to the books, two deep will drive you bonkers. I tried a similar thing many years ago with my excessive numbers of poetry books, and I might as well have just gotten rid of all the ones in the back...I never saw them.
Another solution might be to use the space behind the books for cartons of purely seasonal things like holiday decorations, or for archived papers you need to keep but not get to with any regularity.

Thank you all for the input. I think I need to tell the carpenter to hold off while I rethink my plans. Maybe I need to look elsewhere for a place to put bookshelves and let this alcove be storage for Christmas and oversized items. It's not often you have a ready-made space for large storage items so I hate to just give that up without some more thought. Thanks!

I put my cookbooks 2 deep in my end cabinets and it drives me crazy. I put the ones I use most in front but when I have to get the ones behind it means I have to pull 2-3 out in front to really see and get the one I want. Maybe shelves would be different as long as there's enough height b/t the, but I don't have a lot of height so can't grab it and pull it over the top of the other books once I remove 1 and see the one I want. Unless the one in back is thinner than the one in front I can't just pull it out straight, I have to remove 2 in front to pull 1 out from the back row. But I really don't want to stick my crockpot behind either b/c then I'd have to pull *everything* out to get it!

I'd put the cookbooks on shallow shelves (maybe a wall cabinet? I had one built into the island at old house and thought it was a big waste of space without a deep cabinet on other side, but now I miss the convenience of having cookbooks and phone books 1 deep) in the kitchen and leave the deep storage for big things. I think I have to purge my cookbook collection and find a place for the ones I use often, put the rest in another room.

Could you install roll-outs and put the books on them? Like a 'book pantry"?

I like Lazygardens idea to make a pull out on the bottom
of the closet with your cookbooks.
Here is another idea. I have a cookbook cabinet (2 shelves deep) But I wanted a shelf just for my most used cookbooks.
Also my hubby likes to cook so I needed a place for his favorite books. I worked this shelf into my cabinet design
when we recently added a spice cabinet (the cab on the left) to my existing tv cabinet.
This shelf is only 9" deep x 24" long.
The space you have above your desk looks like about 42"
I think you could fit 2 shelves under your cabinet, even if the first shelf was small and the second one larger.
I measured my cookbooks to get the height of the shelf I wanted.

Just thought I give you another option...
I think two shelves under your existing cabinet would
be nice and handy too. You could still make and additional
pull out shelf as Lazygardens suggested.
The cookbook cabinet is underneith the radio etc...

If you do decide to double shelve here's a suggestion to make the back books more findable: store those books flat, instead of upright liike the front ones. That way pulling out just one or two of the front ones exposes the spines of several of the back books at once.

That's my solution for too few shelves and too many 1000s of books. (Of course this runs into to problems with the dimensions of some books, but if you fiddle with it, it can work.)

Another, more complicated, idea is to make a two-sided shelf insert that pivots allowing the two ranks of books to be placed spines out. Or, if your space is deep enough (and wide enough) you could install a pull-out cab with books stowed on shelves in two rows with their front edges touching and spines out towards the sides of the cab.

Another option would be to build a series of drawers. Turn the cookbooks 90 degrees so the front of them faces toward you. When you pull out a drawer, you will be able to see the spines on the side.

Lisa