System Preferences > Language and Text > Input Sources tab.
Top of the left column, tick (check) "Keyboard and Character Viewer", Bottom of window next to search field, tick "Show input menu in menu bar"
System Preferences > Keyboard > Keyboard tab; tick "show keyboard and character viewers in menu bar"
All the standard diacritical marks and common symbols can be accessed directly from the standard keyboard. ü is produced by pressing alt (option) + U, release them and type U. Similarly é is alt+e, followed by e.
If you open Keyboard Viewer and press the alt (option) key, the location of the marks will be revealed. Note that five of them are highlighted in orange; these are the ones which can be used with multiple letters (acute, umlaut, gràve, circumflex and tilde)
Shift+Alt will show more.
Is there a site or book for hidden strokes?
There are no "hidden strokes".
There are various input methods. You enable them in the system preference indicated by noondaywitch. You can view what characters are inserted by what key with Keyboard Viewer.
Frequently used characters are inserted by pressing the respective key, as such, or with the ⇧ (Shift) modifier. Less frequently used characters can be inserted with the ⌥ (Option) modifier or with ⌥⇧ combination. Other less frequently used characters are inserted with a method inherited from the days of the mechanical typewriter, the dead key. For instance, on the US keyboard, ⌥N is a dead key used to form a few characters commonly used in Spanish.
Due to the large number of charactes employed even in the Roman script and its variants, not all characters can be entered by the same input method. And, of course, the meaning of 'less frequently used' depends on the language. For instance, the character Ą (A with ogonek) is rarely encountered in English, but it is fairly frequent in Polish. Consequently, it is not possible to insert it using the US keyboard layout, while it is inserted directly with the ' key when using the Polish keylayout.
The traditional Mac US keyboard is designed to enter a limited number of characters with diacritical marks (primarily those used in major Western European languages). The US Extended keylayout has a more extensive repertoire of dead keys, which allows for the input of a far larger number of diacritic characters (at the price of dropping Greek math symbols).
Another layout based on the US QWERTY keyboard, the Unicode Hex Input keylayout, allows entering Unicode characters by their hexadecimal codepoint. For instance, to enter the degree sign when the Unicode Hex Input is active, hold down the ⌥ key and type
And a new input method has been introduced in Lion -- holding down a key representing an alphabetic character will produce after a short delay a list of alternate characters (this doesn't work for symbols and it doesn't work in all applications).
For details, read
Note that there is nothing 'hidden' or 'secret' about any of the above. You can find it in any of the reasonably good books on using Macs. If you do a bit of writing, I particularly recommend Robin Williams' "The Mac Is Not a TypeWriter".
pressing alt (option)
Probably a typo. Must mean "option (alt)" -- Macs do not have an Alt key. The Option key (which may be identified by the text label "Option", or by its traditional Mac symbol ⌥, or both) also bears (in smaller type size) the label "Alt", to help when using non-Mac apps.
Switch 101: On Windows, I used to...
In Windows, I used to… On a Mac, I can… ⋮ ⋮ Use these keys as shortcuts: Use these keys as alternatives: ⋮ ⋮ Alt key ⌥ Option key