I honestly think your suggestion in the question is probably the best route.
There are major pros for the players if they pick Bob up as an Aspect
If they have Bob as an aspect, it becomes a good way for them to gain fate points by using that aspect to stumble into danger, as well as giving you a pretty good way to introduce plot hooks. If they want to use Bob to augment their die rolls, they could spend a fate point to gain a bonus to whatever roll/reroll (I'd guess lore or something else applicable that's an applicable use of an aspect, but I can't quite recall). Once you've got them hooked on their new Aspect, take it away, and toss them a plot point so they can go get it back.
If they don't grab him as an Aspect, well, you've got a plot device, but they won't be getting any fate from when he decides to throw them a bone.
Yes, sometimes.
It's easy to miss, but it says on p. 251 (just before the example) that the Discipline roll used to cast the evocation counts as the attack roll. The amount of stress inflicted by a successful attack is equal to (attack roll - defense roll) + weapon rating. In the example, the relevant numbers are:
- Attack roll: 8
- Defense roll: 4
- Weapon rating: 8
So that's attack 8 - defense 4 for success and stress of 4. Since this attack was made with a weapon, now add the weapon's rating: stress 4 + weapon 8 for a total of 12 (less one for Inhuman Toughness).
The math isn't the same at all in the case of losing the opposed roll. If the attack roll doesn't beat the defense roll, then the weapon rating is never factored in and the equivalence breaks down.
So it's not quite "shifts you put in plus your Discipline roll" minus defense, it's the mathematically-equivalent (but procedurally and conceptually different) "attack effect plus weapon rating". If it's easier to remember the first way then that's fine, if you win the opposed attack/defense roll. If you don't win the contest, then thinking of it as "shifts plus Discipline" will just cause confusion.
Here's an example of how the math works when the attack loses the contest:
Harry is fighting one of the Vampire's minions now. He fires off the same evocation that took out the minion's master, putting in 4 shifts—he doesn't want to deal with backlash or fallout just to deal with some poor thrall.
Harry's evocation is a weapon:4 and his Discipline is Great (+4) so he only needs to roll 0 or better on the dice. He rolls 1, so his total Discipline roll is 5. That's enough to control the evocation, and doubles as his attack roll against the minion.
Against all likelihood the minion rolls exceptionally well, getting a +6! Harry's loses the opposed roll and his attack fails, even though he cast the spell successfully.
Now, the numbers look like this:
Discipline roll: 5
Shifts put into the spell: 4
Attack roll: 5
Defense roll: 6
Weapon rating: 4
If we use your math, then the attack is Discipline roll + shifts in spell: 5 + 4 for an attack of 9. Less defense of 6, that would deal 3 stress to the minion.
However, that's not how attacks are resolved! Using the actual math, the attack is (attack roll - defense roll): 5 - 6 which is -1, and since rolls never generate negative shifts (p. 17), the attack is just considered to have failed. The minion won the opposed roll and the weapon rating becomes irrelevant—weapon rating is only added to the shifts generated by a successful attack. The minion suffers no stress, and now Harry is in a bit of trouble…
Best Answer
Style
DFRPG is rather wordy; the manuals themselves are flavorful enough to give a solid sense of the Dresden Files world and its playful-yet-serious attitudes. Sadly, this occasionally obscures the rules a little, but overall it's very well written.
Also, the book's got a lot of art.
General Rules with Examples (35+ pages)
FATE is a weird system for the vast majority of RPGers who are used to d20-style games, and DFRPG contains subsystems unique to itself even when compared to other FATE games. This means that nearly every section has at least one example of how the game looks at the table, and/or how a general rule might be applied in a specific instance.
Examples will pad most of the other categories here, too.
IP Content
DFRPG isn't just about rules: it's a setting based on an established property. Volume Two is almost exclusively Creatures From The Books, and isn't necessary to play the game the way a D&D monster manual would be.
Extra DFRPG-specific Rules (80+ pages, conservatively)
DFRPG is crunchy (crunch = rules and mechanics; opposite of "fluff") for Fate games, and in the running for crunchiest. It splits many stunts off into multiple subgroups collectively called powers (see below for page count). Spellcasting is a relatively complicated system designed to be flexible yet balanced, and altogether occupies more than 80 pages in Volume One. Other subsystems, like the Hunger stress track for vampires and various transformative or magical options, are rolled into other pages counts (largely Powers and Spellcasting).
Campaign and Character Creation (125+ pages)
FATE doesn't just have the party make characters; campaign/setting/NPC design is a group task too. The process of city creation is about 25 pages itself; it's that important. Characters are self-designed rather than class-based, and the Dresdenverse runs the gamut from ordinary humans to werewolves, vampires, and holy knights. So the general rules for character creation and advancement take up 45+ pages (again, crunchier than normal for FATE), while specific options for templates, stunts, and powers are another 55+.
SevenSidedDie points out that just the powers alone "cover a tonne of supernatural abilities, some of them in several varieties, so that you can create new beasties and custom character types from scratch to suit your own Dresdenverse." That should give you a sense of the scope this book is trying to cover.
It's a GM manual too (50 pages)
Between Running the Game and Building Scenarios, they've got it covered for GMs.
Original Content: Baltimore (50 pages)
At least fifty pages of Volume One, and a good chunk of Volume Two, are dedicated to an original setting: a Dresdenversified Baltimore. It serves as an example and a campaign setting, and is very well designed.
Etc.
Throw in a good sprinkling of art (including a full page for each chapter heading), a really good glossary and index, character/campaign sheets for both design and play, and you quickly get a doorstop for a manual.
The book's main bulk comes from examples and taking their sweet time to explain things. While it's good that they try to be so explicit, I think that often the book's style (while entertaining, readable, and good for getting into the mood of the game) gets in its own way when the rules should be presented clearly and concisely. A good solid editing with that in mind might have dropped the page count noticeably.
FATE is simple, but contains complexity
For comparison, the generic FATE Core book coming out next month will probably be 300+ pages. It's stripped of setting, but features sections on philosophy and how-to-make-your-own-setting content. Despite the mechanics of FATE being surprisingly light, teaching someone how to implement FATE without either losing him in vagueries or tying him to unnecessary rules is not a simple task--especially in a book rather than face-to-face.
The DFRPG manual tries to include setting design and game philosophy (for those who are unfamiliar with FATE from other sources) as well as dedicating a couple hundred of pages to its own intellectual property.