I assume when you say primer you mean easy "reader"?
Here is a good primer of Medieval Latin. Easy selections. Poor notes and Bad dictionary though.http://www.amazon.com/Primer-Medieval-L ... 240&sr=8-1
Beginning Latin Poetry Reader is good.http://www.amazon.com/Beginning-Latin-P ... 299&sr=1-1
Lingua Latina can work as a Primer.
Google Books--consult edonnely's list--has many primers for download. I printed a few and made them spirals.
I like Pharr's Aeneid
, though it is a little dense.
Having reread your post, it has occurred to me that you may simply mean "textbook" when you say primer. If you are self-tutoring, pick up Lingua Latina. http://www.amazon.com/Lingua-Latina-Fam ... 884&sr=1-1
Hang on to the Cambridge as well.
If you find yourself flying through Lingua Latina, you may want to pick up a grammar book for reference.
When a true genius appears, you can know him by this sign: that all the dunces are in a confederacy against him. ~Swift