No easy answer since each gun/bullet/charge is different; i.e. you gotta' test. Published cartridge OAL (is that what you mean by "book length"?) is nearly always associated with making rounds short enough to fit into a magazine. If you single feed a bolt gun, using a longer cartridge is acceptable. In fact, some of my competition cartridges are so long that I can't extract an unfired round without removing the bolt.
When you talk about seating distance relative to the lands, most folks measure CBTO, Case Base To Ogive, usually measured with a special tool like the Hornady Bullet Comparator or similar. If you don't have one, you might consider getting one if you intend to experiment with seating depths. Measuring overall length is a fools errand because bullet length varies too much and the important thing is the CBTO.
In the interest of full disclosure, it should be noted that most bullet seating dies contact the ogive in a different place than the Comparator tool. The place the ogive touches the lands is what we're interested in, but unfortunately, these contact points are at three slightly different spots somewhere on the ogive (curved part) of the bullet. That's not ideal, but it's much better than using the bullet tip as a reference point.
Although there are many ways to skin a cat, I typically start load development by shooting 7 or 8 five-shot groups at various charge weights with a "jump" of .010" off the lands. After finding the best charge weight, I load and test 6 or 7 five-shot groups at various seating depths starting at a .020" jump all the way to .010" jam. Later, I might fine tune and verify the charge weight at the best seating depth.
However, after reading the seating write-up on the Berger website where they recommend testing VLD bullets over a much larger range, up to .120" jump, I now test VLDs at longer jump distances. To my surprise, I have found some of my VLD loads work best at seating depths as much as .080" jump. I would have never tested seating depths with that much jump before.
I know some shooters who always jam their bullets, often as much as .020" and don't even test any seating depths which involve a jump. Others, especially competitive shooters or hunters who might have to extract an unfired round, shy away from jaming bullets for fear that the bullet will stick in the bore and dump powder inside the action.
Some folks say test various seating depths but stay away from just kissing the lands; i.e. .000, because slight deviations in the way you load ammo will mean some jam a tiny bit and others jump a tiny bit producing inconsistencies at the target. Others poo-poo this notion and claim .000" is perfect for them.
Bottom line: Testing is the key, along with owning a few specialty measuring tools. They aren't too expensive, but if you intend to test various seating depths in the search for more accuracy, they're indispensable.
