Your could try this linear regression:
Quality_of_link = alfa + B1*Number_of_links + B2*User_base + error term.
To determine parameters (B1 and B2) for Dependent Variables (Number_of_links, User_base) you could use historical data (number_of_links; user_base; quality of link) and estimate the values of the parameters by running a linear regressions. You could do this in a statical program. Good statical programs include R-project and SPSS.
Important in this respect is the objective way to determine the Quality_of_link. I think you could do a test by rating a number of links, preferable by the targeted audience of you site. Then use the average value given on a scale (e.g. 0-100) to the links.
After you have run the regression in you test phase your can use it in you final model. This would then be: Quality_of_link = alfa + B1*Number_of_links + B2*User_base. You could then use say above a Quility_of_link above 70 is a good link and higher than 90 best link.
For good textbooks it will be difficult to point you to a particular book which I haven't read myself. I would first recommend using the knowledge you already have an use the internet if some knowledge needs to be refreshed.
Hope this helps. Success with your project.