I have some text books that I want to digitize to computer files. They are
mostly reference books for graduate studies in computer science. I own hard
copies of those books but I want a digital archive of them so that I can
index and search.

I know what I have to do: 

1) scan each book page by page,
2) run ocr (optical character recognition),
3) save them as ascii files.

But I don't want to do it myself (time-consuming!). I want to ask: is there
any company that provides this kind service at a reasonably cost?

Hi Modular,

Thanks for asking about having your textbooks scanned. I have found a
number of companies that will do OCR scanning. I have been unable to
find any really helpful information on their costs within the hour it
takes to lock a question (as it's Saturday, and I doubt I'd hear back
til Monday), but each does have contact information, and since your
project is an unusual one, it would probably make more sense for you
to contact them directly. I say 'unusual' because of the small scale
-- most of these companies typically service large corporations,
libraries, archives and database producers.

To start out, I went to the Google Directory Business services
section:
http://directory.google.com/Top/Business/Industries/Information_Technology/Imaging/

Browsing through the companies that populate this category, I have
narrowed the list down to those that definitely have document scanning
to OCR:

Crowley Micrographics
http://www.CrowleyMicrographics.com/
Located in Frederick, MD.

Micromedia Imaging Services, Inc.
http://www.imagingservices.com/
Select "Services" ---> "OCR and manual data entry" to find out more. 

DocuScan USA
http://www.docuscanusa.com/
"Document Scanning & Conversion to Word Processing (OCR)
     Get all those documents into a format you can easily index,
     retrieve, edit and share. Documents in sizes up to 11x17" are
     scanned into standard image translation formats. These can be
     used as is in most Electronic Document Management (EDM)
     systems. Or have us OCR convert them to your favorite word
     processing format. We scan and index single- or double-sided
     documents in order so no further organization is necessary."

Scan Store
[http://www.scanstore.com/Services/]
Claims to be "the [N]et's most extensive website for document imaging,
document management, OCR and forms processing solutions." Who can say?

From my own experience with optical character recognition I offer the
following unsolicited caveats: First, OCR is a developing technology.
It's not foolproof, and you may need to pay to have the files
proofread. I have encountered many scanned documents that conflated
particular sets of letters (i+n=m, for example). Second, a year and a
half ago I held an internship at JSTOR, an online journal archive
(http://www.jstor.org). They do OCR scanning of bound journals, but
must have them unbound before they can do so. Be prepared to have your
textbooks re-bound if you want to keep the paper versions.

I hope this is helpful. If not, please tell me how it's not helpful,
and what else you need to know.

Thanks
Libronaut
Primary clients are

thanks for your detailed answer! 

though i found out this solution to be too expensive... i need to do
it by myself (page by page) after all :(

May I suggest Abbey FineReader for this purpose? I've not used it
myself, but it is highly recommended by those in the know, namely,
'bookwarez' pirates who scan in physical books and distribute
electronic copies for general consumption. I of course refrain from
such nefarious activities in the #bookwarez channel of irc.nullus.net.

bookface-ga, thanks for the useful comment from the "know". 
I will check it out.

But, how does FineReader compare to Omnipage, despite the prices?