"Guide to Inbreeding and Line Breeding"

Many breeders use line breeding as a tool to improve and learn more about what is behind their lines. Done improperly line-breeding can cause issues, but an educated breeder should know his or her dogs an their pedigrees, and know what is behind them. Tight line-breeding is the quickest and easiest way to set “type” or the general look of a dog and a breeder's lines. By breeding like to like, you end up with cookie cutter dogs.
Tight line-breeding from healthy dogs also will secure healthy lines of dogs will secure healthy lines of dogs, as you are not doubling up on faulty genetic material. Breeders occasionally want to get rid of a certain gene, or genes, that cause health problems. A tight line-breeding will force any faulty genes to be expressed, and they can choose to not continue breeding that particular pair of dogs, ensuring that future generations are healthy and happy.

I found this really good article too!

https://davidcavill.wordpress.com/a-beginners-guide-to-inbreeding-and-line-breeding/