I actually believe the Kaplan book covers the Biology section VERY well! But there are always a couple of weird questions on the DAT, that you wouldn't know unless you memorized your Biology book.
If you have a limited amount of time (3 months or less) and need to do very well on the DAT, then memorize the Kaplan book.
For example, I had a question on my DAT concerning human physiology, and I had NO idea what the answer was! I taught physiology at my undergrad for 2 years and I *still* had no idea what the answer was.
In all honesty, it just isn't worth it to put in hours and hours of time just to nail those few questions. If you memorize the Kaplan book then you'll still do well, even if you miss a couple of those weird questions.
All in all the Kaplan book is a GREAT deal, especially when you analyze its cost. I remember purchasing it for $45 or something, which is amazing, considering it and TopScore were ALL I used to study, and my scores were pretty good (above 85%ile in every subject, and above 96%ile in two subjects).