Nobody builds bookshelves for book people. Commercial shelves are always too deep and high, which wastes a great deal of space. Below is the solution I came up with for shelving way too many books in way too little space.
Using nominal 8" wide (i.e. 7.5" wide) and nominal 1" (3/4") thick stock, materials per shelf unit are:
2x 8' boards
2x 30" boards
8x 28-1/4" boards
about 70 nails or drywall screws to taste
3-4 asst'd small mending plates or angle irons to taste
primer and paint, if you're using it
You can save a little money by cutting the lumber yourself, but don't try it unless you're sure you can make all the shelves come out exactly the same length. If you're not handy, just go to a lumber yard that will cut wood to size and give them a list of how many boards you want in what sizes.
If your nails or screws are longer than about 2", the work will be harder and the boards will be likelier to split. If they're an inch long they're too short. The mending plates or angle irons are to fasten the shelves to the wall. If you have fine hardwood paneling on your walls, use some other method. Otherwise, it's between you and your landlord. Remember that love may come and go, but a molly bolt is forever.
It's easier to sand boards and apply primer coat before the shelves are assembled. We went for plain raw pine. It's up to you. But if you're going to do it, buy the primer and paint at the same time you have the lumber cut, then go home and get to work on it.
If you let your freshly-cut lumber sit around for weeks and months before you get assemble the bookcases, you'll have trouble. There's tension in wood, and when you cut it that tension is released. Your boards will start warping. Get them nailed up into a shelf before they have time to think about it.
All joints are butt joints. Yes, really. They're perfectly adequate as long as you don't make the shelf much wider than I've indicated. If you need shelves to fill a space 48" wide, make two bookcases.
-- 30" -- _______________________________________ _3/4" | | | | 10" | -- 28-1/4" -- | |_____________________________________| _3/4" | | | | 10" | | |_____________________________________| _3/4" | | | | 10" | | |_____________________________________| _3/4" | | | | 10" | | |_____________________________________| _3/4" | | | | 10" | | |_____________________________________| _3/4" | | | | 10" | | |_____________________________________| _3/4" | | | | 10" | | |_____________________________________| _3/4" | | | | 10" | | |_____________________________________| _3/4" | | | | 10" | | | | --------------------------------------- -3/4"
The boards at the top and bottom of the case are 30" wide; the intervening shelves are 28-1/4". The distance from the bottom of one shelf to the top of the next is 10". There are eight shelves and nine such spaces; (9 x 10") + (8 x 3/4") = 96", the exact length of the eight-foot boards.
What you do about baseboards depends on the baseboards you have.
You save a lot of time and effort if you make a bunch of bookcases at once. Lay your eight-foot boards flat on the floor, side by side, and immobilize them. Pipe clamps will do. Masking tape's better than nothing. Measure and mark where the top and bottom of each shelf will be, then take a long straightedge and draw lines across the widths of the boards to show where the shelves will go. This will be immensely helpful later.
I find it easier to assemble the bookcases if I leave the top and bottom boards for last. Getting the first shelf nailed in between the upright supports is the hardest part. The best solution is to have several friends around. Make sure you buy beer and pizza for them.
I favor three screws or nails per shelf per side. Overengineering is good. Overengineering is our friend.
Don't square up the bookcase unless you have a square house. Otherwise leave in a little play. It'll all firm up anyway once you've nailed it to the wall and filled it with books.
Nail or screw a few small mending plates or angle irons to the back of the bookcase. Stand the bookcase up against a wall and nudge it into position. Fasten it to the wall through the holes in the mending plates and angle irons. I favor a discreet 4" drywall screw myself, and I hope you don't gotta problem with that. People whose walls aren't made of chalk and cheese may want to use smaller fasteners and some of those nice screw-in anchors.
You can fasten the bookcases to each other for additional stability. If you life in California and you're really worried about stability, don't build eight-foot bookcases.
You get about 21 feet of ten-inch-high shelving space per bookcase, and the bookcases don't stick out into the room as far as conventional bookshelves. There's an additional 30" of horizontal space on top of the bookcase, if you want to use it. In seriously cramped areas you can use six-inch-wide stock throughout, and you'd be surprised at how stable most books are in such shallow space.
The ten-inch-high shelving space will accommodate most hardcover trim sizes. You'll want to have at least one other bookshelf in the house to hold your oversize books.
The shelf positions don't move. Big deal. Movable shelves are for interior decorators.