PP, you're right, so I'm glad you stopped before offering your own definitions. But unfortunately the books can be just as confusing:
Advanced Marathoning -" tempo runs in which you run for at least 20 minutes at your lactate-threshold pace. This corresponds closely with your current 15K to half marathon race pace. For most marathoners, this pace range corresponds with about 82 to 91 percent of maximal heart rate."
The Competitive Runner's Handbook - "Lactate threshold training is primarily at a pace slightly slower than 10K race pace (15K to half marathon pace). Tempo runs are the most effective way to improve lactate threshold."
Note that whilst the paces stated are the same range, the former has a statement on duration and pace - tempo run, at that pace, for at least 20 minutes - whilst the latter has a simple statement - tempo run. From the latter you wouldn't know that this meant a tempo run at that pace for a certain duration. But where did "tempo run" suddenly come from? And why is it that range? Well, it's that range because the titles are Advanced and Competitive, and are still using the Daniels' version 1 of tempo pace being fastest pace for 1 hour, observationally based on the groups he was coaching. So, you go back to Daniels, and now you find out that he's updated his definitions of tempo, because he's realised that he was coaching Advanced Competitive runners, and the rest of the world isn't always that good but still wants to improve.
And you're back to square one on Lactate Threshold.
Advantage number one of the internet over the books is that the science is the latest, unfortunately you have to pay for the best of it. Books are still in the realms of VO2max being the most important factor in distance running - without even being able to explain exactly what it is. That said, I completely agree that they are a very good place to start. I'd just go a little further backwards and start with a basic Exercise Physiology guide. From there you're armed to go surf the big, scary internet and make your own decisions on what is clean and what has been tainted by evangelism.
Or if you don't want the outlay:
For initial definitions of all the above, go to www.brianmac.co.uk.
For VO2max google Billat - she publishes her papers online.