Hi, I made this instructable to show how to add some solar cells to your e-reader, so that you will never need to charge it again.

Update: I replaced the Schottky diode with a MAX1555 Li-Ion cell charging IC. Thanks go to moldboy and Kohaku for their input on making the charging circuit safer.

What you need :

An e-reader, I use a first gen Kobo, but I'm sure this is also possible with any other.
Some solar panels, like these. Perfect size and voltage and decent power too.
A Schottky diode , you can get this at any shop that sells electronic parts. You need a diode to prevent discharge of the battery. The advantage of using a Schottky diode is the lower voltage drop.
A MAX1551 or MAX1555 IC. They operate with no external FETs or diodes, and accept operating input voltages up to 7V, so very easy to implement. Here is a datasheet. I ordered a sample from the manufacturer.
A SOT23 to DIP breakout board. You need this, because it's quite hard to solder wires directly to the MAX1551/1555, since it's so small. I got this from eBay.
A small capacitor, to make life a little easier for our MAX1551/1555. I used a ceramic one I had laying around.
Some wires, a soldering iron, solder, a dremel and padded double sided tape. A multi-meter can also come in handy.

Here are some minor technical details:
This e-reader (as most of them are) is powered by a 3.7V li-ion polymer battery. These kinds of batteries need pretty tricky charging curves: fast at first and trickle charge till full, then the current stops, since over charging can be dangerous. For this reason I replaced the Schottky diode I used before with a MAX1555 charging IC. A Schottky diode also works, but it can damage the battery in certain circumstances.
The specifications of these solar panels claim that they can do 80mA, but after some measurements I came to the conclusion that in reality they will only reach about 50mA. Adding the two panels together we come to 100mA. This means that they should be able to charge the battery from nothing to full in about 10 hours. However, the charging current is controlled by an IC and will be lower during the later half of the charging cycle.

Step 1: Taking Apart the E-reader.

The first step is to open the e-reader, but since it is not held together with screws, this can get kind of tricky. What I did was to take a knife and pry it in between the two halves of the casing at a corner. After this came apart, I worked it all the way around the entire device until it came apart. Watch out for the double-sided tape that is between the top cover and the screen.

Detach the battery wires from the main board and remove the battery.

Remove the four screws and carefully lift the PCB with attached screen from the rear panel. There is also some clear plastic behind the main board, don't lose this.

Step 2: Cutting a Hole in the Back Cover.

Decide where you want to put the panels and mark an outline. Cut a hole with the dremel and make sure the corners are nice and straight.

Step 3: Wiring the Solar Panels Together.

Wire the two panels in parallel; this means wire the pad marked '+' to the one marked '+' on the other panel and the same for the '-' pads.

Step 4: Putting in the Solar Panels.

Use some strong padded double-sided tape to attach the panels to the metal shielding. Make sure the solder pads you want to connect end up at the top and are not covered in tape. The kobo has a little square spacer in the center of the back cover; cut it off the piece you removed from the casing and glue this back to its proper spot.

Step 5: Soldering the MAX1551/1555 to the Breakout Board.

The MAX1551/1555 is in a so called SOT-23 package, which means it's really tiny. To make it a bit easier to wire, you can use a breakout board; I used a SOT-23-6 to to DIP adapter I found on eBay. Soldering the IC to the adapter can be tricky and requires a little bit of patience.
You can follow this guide, but I'll summarize here:

Place a small amount of solder on each of the pads (the guide linked above does this a bit differently).
Place the IC with a pair of tweezers or a small clamp on the board and solder one of the legs.
Gently put a little pressure on the IC and heat all the legs so that solder will flow between the board and the legs.
After letting it cool for a second, we can check the connections by measuring the resistance between the IC leg and the soldering holes in the board.

You can also fill the holes with a little bit of solder now to make it easier to attach the wires later.

Step 6: Wiring the Panels to the Battery.

Solder wires to the positive and negative terminals of the solar panels, with the capacitor in between (I used a ceramic one, so it doesn't matter which way it's connected. If you use a different kind, check the polarity).

Solder another pair of wires to the positive and negative terminals of the battery; you will have to cut away some of the tape. After you are done soldering, put some tape back so it won't short out against the back shielding of the device.

Put some tape beneath where you want to put your IC so that it doesn't short out against the EM shielding.

Now you can solder the wires from the battery and the solar panels to the IC. Follow the nice schematic attached to this step: the negative terminals of both the battery and the solar panels are wired to pin 2 (GND), the positive terminal of the solar panels is wired to pin 1 (USB) and the positive terminal of the battery is wired to pin 5 (BAT).

Place the board somewhere where it's not in the way of anything when you put the e-reader back together and put some more tape over it to fix it in place.

Step 7: Put the E-reader Back Together.

Remember that clear piece of plastic you found between the PCB and the EM shield? Put this back to cover the shielding and the wires/IC.

Put the main board with display back in its proper place. Be careful and double check the position of the board you just soldered, so that it's not in the way of anything. Screw the four screws back in.

You can now snap the back and front of the device back together and see if it works. I tested mine out by first checking the battery (it was at about 1/3rd) and then finding a nice bright sunny spot to charge. After an while I checked the battery level again and it was at the next level (about 2/3rd).