No, I disagree. A book isn't your best bet, but a well made DVD can walk you through it and explain what you should be looking for and feeling as you do the positions.
Perhaps, I was fortunate to get some really good DVDs and perhaps, my athletic background helps, but I've had nothing but very positive results from practicing yoga at home with instruction from a DVD or this one show that I used to record.
Most people tend to be only slightly self-aware when it comes to body mechanics. Ask any physical therapist or dance instructor - people can go through their entire life holding their head at an angle and believe that it's "right". They have little kinesthetic sense unless, like you, they are experienced in athletics and have received prior coaching.
I won't claim that you can't get SOME benefit from videos; only that you're not getting the full effect. This might be fine if all you want is basic relaxation and breathing practice, but if you're going for the higher levels of practice every posture, every breath and every thought are going to have to be "right-on".
Again, I'm not as experienced in yoga as I am in Taijiquan, but their basic postural and breathing techniques are quite similar. All too often I've seen people self-taught in Taiji from a DVD and what they're doing is often not only posturally and energetically incorrect but can also be dangerous.
Instructors provide many things - experience, a watchful eye for dangerous practice, motivation, hands-on correction (having an experienced set of hands PLACE you into a position can be vital) and a role-model to emulate. Four out of these five things I don't believe you can gain from a video.