Your daughter actually sounds normal to me. I wouldn't be concerned about her not walking or not showing interest in walking until she's closer to 18 months old. The reason is because she's sitting unassisted and because she's mobile. She's able to get around by crawling army crawl and she CAN get up on her hands and knees.
You have to toss those stupid books. The milestones are just loose guidelines, not solid Your Child Must Do This By This Time Rules Or Else. Give her time. From your description it sounds like she's rocking on her hands and knees, testing out the "pre-crawling" on her knees but some children skip that stage entirely.
If you're honestly concerned, talk to your pediatrician at her next appointment. The pedi will do a few simple tests with her. But what you're really looking for are her fine motor skills and if she can understand what you're saying and if she can follow simple directions. Is she talking at all? Trying to color? Self feeding? Clapping? Pointing? Waving hello and goodbye and doing other signs?