Disclaimer: Sorry that I haven't taken pictures of the process of building. I hope the description and the sketch help. And sorry for my English.
Bookshelves don't need to be bulky, they do not have to be deep. Let us consider how much space does a book really need. If it is standing on minimum 50% of its width, it won't fall down, would it? And how wide can a book be? The majority won't have more than 20 cm, so a shelf of 10 cm depth should enough for most of your specimens. The others can go on an extra place.... Imagine, if a shelf has a depth of 10cm, the books will look like nearly coming out of the wall, they are easier to remove, easier to clean, and anyway there is less space for dust.
Other pros: it takes the minimum space (physically and visually), it has nearly no screws in the wall so it leaves nearly no trace when taking it off, you can readapt the altitude of the boards if you need it, and it really looks great.
In my case, I love books, have a lot, but little space and no understanding for huge bulky furniture. I saw this kind of construction in the house of my Grandfather, made in the seventies DIY design, and well, have built 5 versions so far. You see them in the pictures, in historic order: first in Berlin (on this page), two in Mexico City, two in Lima.
Step 1: The Concept
Ok, the logic is the one you see in the sketch.
The construction is based on the idea that the sustaining bars go trough the boards, so that they hold all the vertical weight. The boards lay on pins which go through that bars. That way, you can adapt the height of a board making new holes for the pins. I would not recommend doing it too much, but in same cases it has been necessary.
I usually have made my shelves from wall to wall, so I could use at the edges an extra support (headless nails) and it looks better. But they can be interrupted (as you see in the variation on next page).
How much boards are you going to use? Consider how high your books are. I usually go between 20-30cm. And well, you can calculate 50-70 books on a meter, but you can also put other stuff on your shelf. How many bars? I used always two, but the newest one (last page) had a length of about 3.5 meters, so I used 3.
I recommend boards 10 to 12cm deep (have made from 9-15), 1-2.5 cm thick. The bar can be 2-4cm thick, depending of your boards (less thick and deep the plank, the smaller has to be the bar). The bar can also be a metal tube (like in my second variation, on this page) - in that case the pins must be metal sticks. I always used pins about 60% the measure of the board. In the lightweight versions (next page) I used wooden bars with headless nails as pins.
And remember: you have to measure quite exactly. All has to fit quite tight, otherwise your material will bend and you loose stability.
Step 2: Preparing...
1.- Prepare your boards. Cut the exact width. I usually treat them, after sanding, with a mixture of clear varnish with linseed oil, the color is a slight yellow as you see in the picture.
2.- Prepare your bar - it has to fit really exactly from floor to ceiling. It is recomendable to insert on both ends rubber tops, so you can insert the bar with pressure to chock in its place. Measure the length of the bar considering that tops. Drill the holes for the boards into the bar, centered, at exactly the same height and with the diameter of your pins.
3.- Drill into the boards the holes for the bars. In case of the 2 bars version, drill at 30 percent distance from each side (for example if 2m wide, measure 60cm from each side, leaving 80cm between the holes). This holes have to be nearly exact the diameter of your bar so it passes tight (I have used oil to make it pass) - and they have to be the same position on every plank! Otherwise you will have quite a fuss passing the bar trough all the planks. It helps to mark, with a pencil on the edge of the boards, the order and the orientation you want to finally assemble them.
4.- Ok, now the moment of truth. Lay your boards vertically, so that you can pass the bar trough them with the shelf lying on the floor. I wish you luck with the symmetry of your holes. Once done, put the rubbers.
5.- And now hoist the sails! Lift it up. We rammed a door frame in the last variation, due that half a centimeter of it was in the way...
6.- Now liften the boards. Start with the highest one, and fix it on its position on the bar inserting the pins. Continue downwards.
7.- Ok, with luck and maybe some corrections, you have the shelf standing! It should hold the weight. But it isn't stable now.
8.- Lets secure it, so it does not came forward nor vibrate. It is enough to fix only one of the higher boards to the wall, I recommend screwing a L-shaped metal piece in two equidistant positions, for example behind the bars. The great thing is that if you need to take the shelf off, you just leave 2 holes in that wall.
9.- If you want, you can add an extra support at the edges, if they touch a wall. I used it to avoid bending if the plank was too narrow or the distance wall to bar to big. Take long nails, cut the heads off (for example bending them in both directions with pliers), drill a hole exactly under your boards of the diameter of the nail, and insert the nails in the wall to about 70%, so that the board lays on the rest (it is amazing how much weight a nail can hold as long you don't leave space to bend). Well, this are a lot of holes on both sides, but they are easy to spackle when removing the shelf.
Step 3: Ready
10.- Fill it !
As you see, you can variate a lot the details of this construction.
The first one, and specially this last on this page made 18 years later, are my favorites. This one is even only half the height of the wall, because we had no other place to put that other long shelf below - and they combine quite good.
Now you have a practical, good-looking bookshelf, and you won't feel that you friends the books shut themselves away in the darkness of bulky furniture.