Books, especially multi-volume editions that you have always wanted to read but have never had the time to are a no. You won’t have time on the Camino either. Spanish dictionaries or course books will both weigh you down and get you down. Just accept the simple truth that you won’t speak fluent Spanish after three weeks holiday and stick with downloaded applications with glossaries on your smartphone. Don’t take food, thermos flasks, camping stoves and tents. Leave at home that heavy camera, especially the tripod; I carried both with me only because I needed photos for the guidebook, otherwise without any pangs of conscience I would have abandoned them in the first ditch.
Restrict your luggage to a minimum. To successfully complete the Camino, you need: 3 cycling or running t-shirts, 3 pairs of cycling or running trousers (two short pairs and one long pair or the other way around depending on the season), 3 pairs of cotton/lycra dark underwear and 3 pairs of dark cycling socks. I highly recommend renouncing the pastel lace underpants as they don’t look good drying out on the panniers.
In July/August you need 3 sweatshirts and 4 any other time of the year that can be worn as layers. In June or October, you also need to pack a base layer top and trousers, preferably made of wool as in the morning, even in the full sun temperatures might have dropped to 8 degrees.
You need a pair of cotton leggings/trousers and t-shirt to sleep in (warm ones if you travel in June or October; alberges are heated but not excessively) and one change of cotton clothes and comfortable shoes for the evening.
Cycling gloves, a waterproof jacket, waterproof over-trousers, a headband, comfortable cycling shoes and helmet are an absolute must. Also buy a first aid kit from an outdoor shop and always keep it at the top of your pannier. Pack one small and one large light travel towel, flip-flops (shared showers) and try to compress your toiletries by pouring shower gel, shampoo etc. into small travel bottles. I hope that you will be more successful at that than me as I always finish up with an additional kilogram of absolutely essential toiletries.
Ladies don’t pack make-up because you won’t need it; after two days cycling in the Spanish sun, you will look naturally good. Even my extra kilogram of toiletries (which I’m not proud of) does not include makeup.
Like everyone else you will need to do your laundry every day, so pack a small bottle of liquid travel soap from an outdoor shop (if you hate hand-washing, for an additional charge you can use the washing machines in alberges; pretty often they also offer laundry powder).
You need an anti-bedbug sheet and sleeping bag (you can buy a special anti-insect spray for treating sleeping bags in outdoor shops) that can fit into your panniers, warmer ones for June or October. A sleeping mat is not necessary, but I carry a small mat to sit down on as I like to eat al fresco during the day. I don’t set off on my journey without a Swiss army knife; recently I also packed a small plastic container and travel cutlery for packed lunches. Money belt for cash and cards is a must. In albergue never leave your money, phone or GPS in panniers. At night have them inside your sleeping bag.
For your bike, you need a repair kit, a spare inner tube, lubricant, a pump, a multifunctional tool and a tool to take off the pedals. Remember that the latter tend to stuck and are impossible to move without a special tool more powerful than yours, so if you travel by plane and have to take them off, before your travel, ask staff in any bicycle shop to loosen and tighten them for you (the same before your return).
Other than this, you should pack three Michelin Spanish Regional maps: 571, 535 and 575, PDF of this guidebook and a waterproof cover for them. Please take nothing else. Your luggage shouldn’t weigh more than 13 kilograms; it would be perfect if you could make that 10 kg. Remember that you will carry a bottle of water and probably some food for the day that will add extra weight to your pannier. Panniers have to be fully waterproof. I use only back panniers of 40 litres combined capacity, but you may prefer to add a front one as well. A handlebar bag with a waterproof guidebook/phone/map holder might be useful although I made this guidebook as practical as possible, so you don’t have to pull it out of your pannier every 5 minutes.