I do some Schematic and PCB design: mostly micro controller based systems sometimes analog circuits.
There is a lot of interface handling involved Infra (remote controllers), RS232/422, parallel, USB, RF… (these are only the physical layers there are a lot more protocols)
These things are a bit harder to do than cooding not because it’s more difficult, but you need more then just a PC to do it… (you cannot just download an IC or resistor)

There are some good pages on the net. There’s a lot of info on simple and even on complicate circuits. There are some subjects that are hard to find information about. (this doesn’t sound right)
Van néhány témakör, amiről nehezedd információkat találni, legalábbis jó leírásokat. Például digitális jelfeldolgozásról sokkal kevesebb szó esik, sokkal kevesebben értenek hozzá igazán.

Here are a few links I found useful:
beyondlogic.org
avrfreaks.net/
Some Manufacturers:
Atmel, Texas, Maxim, FTDI,
Cypress, Analog Devices, HB …
I wont list all of them, mostly because i could not, but if you want something like that than I Googled “semiconductor manufacturers” and the first hit seems good.

Sometimes it’s hard to find good if any datasheets for your components.
Well google is still your friend, but here is some links worth mentioning.
alldatasheet
datasheetcatalog
epcos ferrite catalog
You can of course look for datasheets at the manufacturer and also at stores:
Farnell
Lomex (HU)
SOS (HU)
RET (HU)

Good Development for everyone!