What is the best time of the year?
Everything out there coincides in suggesting spring and autumn as the best seasons to do the trip. The specific months would be all of end of April, all of May and the first half of June in spring. In autumn it would be the second half of September and the first half of October.
First there is the weather. The chances of rain are relatively low, the days are long (specially in spring) and it's not too cold or too hot.
Then there is the crowds. Any closer to summer and you'll find almost everything full.
Where do I start?
Anywhere along the trail is ok. If you want to gain your Compostela then you'll have to start at least 200Km before reaching Santiago.
My suggestion is that if you have the time the longer the better. It takes a few days to get into the rhythm so if the journey is too short it'll finish just as you're starting to enjoy it.
The most documented part is the one within Spain. It is also the part that has the most infrastructure catering to pilgrims. Most guides and books, cover the trail from Roncesvalles to Santiago which has become the de facto standard.
I personally started in St. Jean Pied de Port. Many a source mentioned it was worth while to travel that extra stage because of the views crossing the Pyrenees. The weather couldn't have been better for me, so it was visually the most rewarding part of the whole trip. The same as everybody else that has done it, St Jean Pier to Roncesvalles is right at the top of my recommendations.
Bear in mind though that it is an area where the weather is very unpredictable, so you may end up spending a day in the rain wading through fog as thick as pea soup. Quite a few people I encountered along that part of the trail had turned back just the day before because the weather was so bad.
How much will it cost?
Staying only at albergues and buying your food at the market instead of eating in restaurants or bakeries, you can probably get by with about 15 to 20 euros a day (as of 2005). On the other end of the scale, if you stick to the bigger towns and cities, you can book into 3 to 5 star hotels and eat in upmarket restaurants sending your daily budget into the hundreds.
At the busiest time of the year, it would be risky to count on being able to find an available albergue at the end of every stage, specially since pilgrims on foot have a preference over those on bikes
Most people combine albergues with a few nights at 2 and 3 star hotels, "casas rurales" or "hostales". You'll pay 20 to 50 euros for a room with its own bathroom . The final mix will depend on your budget and you need for creature comforts.
What books should I get?
I bought 3 books: El Camino de Santiago - Guía Práctica del Pergrino by José María Anguita Jaén, Guía del Camino de Santiago 2005 by Antón Pombo and El Camino de Santiago en Mountain Bike by Juanjo Alonso.
The first two are actually written for people who plan to walk not ride, but either of them is more useful than the one written specifically for people on mountain bike. Mr. Alonso's book is too vague for my taste. Most of the book is devoted to his experience on the trail, not to practical information.
Of the other 2, the one by Antón Pombo is by far the best.
As far as I know these books are only in Spanish.
When should I take the road instead of the actual trail?
Invariably you will see suggestions in guides, books and sites to take the "alternate route on the road" when the authors consider a particular section "unridable".
My suggestion is to take the trail all the time. Except for moments when you are riding a meter or two parallel to the road, the scenery is considerably better on the trail. You'll meet many more people and its the only way you can get a true sense of what "real" pilgrims went through in centuries past. Depending on your ability and fitness the amount and length of the occasions you'll have to push the bike will vary but I doubt there will be any unpassable section. There is also the issue of safety. Some of the roads you would have to take if you leave the trail can be down right dangerous. That said, a lot of people do a considerable part of the journey on the road. In the end it is a totally personal decision.
How much time will it take?
This is very relative and is covered in depth in the preparation section of this site.
Where are you going to start? How fit are you? Will you be traveling alone or in a group? How much time do you want to spend site seeing and not actually riding the bike? The answers to these questions will affect the time it will take to make the trip.
As a rule of the thumb you can figure 40Km a day average on the low end and 110Km a day average on the (very) high end. The operative word here is AVERAGE. Your shortest day will probably be half of the longest due to the differences in the terrain.