Where can I read C99 official language standard manual/reference/specification online?

The standard itself is here^(a) (with TC1, 2 and 3 included), the rationale document is here.

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^(a) Actually, it's the final draft but is still useful because:

• the changes from final draft to published standard are usually minimal to none; and
• it's free unlike the actual standards :-)

The official standard should not be available online — ISO and its national standards bodies would be unhappy about that. You can obtain an official electronic copy from the ANSI store (http://webstore.ansi.org/) for $18 $30 (or thereabouts — it was $18 when I obtained the C99 standard, but that was quite a while ago now; it was $30 for the C11 standard). Or you can go to the web site linked in other answers (and via Wikipedia) — namely http://www.open-std.org/jtc1/sc22/wg14/ - and obtain almost official copies of the standards documents.

Here's the link to the standard N897 C Standard Rationale and here's the full documentation N1124 as set down by ISO committee, here's a wiki detailing about the standards