In my opinion every kind of material could be used to sketch or create art, but it is true that the right brush and the right paper give you a better change to reach the goal (in more professional way).
So, thinking that this will be useful to everyone anyway, I wrote here the list.
Buy or do it yourself two sketchbooks at least: a little, cheap sketchbook (never mind the brand) useful for quick exercises and a better sketchbook (find here a really good brand: https://www.facebook.com/laloran.booksketch ) for more complex/watercolor sketches.
Watercolor paper 300g/mq Arches or watercolor moleskine sketchbook for travellers. Truly good brands for handmade paper are: Lorenzo Santoni by Fabriano, very good and cheap, and Hahnemuhle paper.
Every kind of paper is good if you are just sketching. I'm fine with Fabriano sketchbook, hardcover, A4 landscape, 110g.
Moleskine watercolor sketchbook cm 18x12 + waterbrush Pentel, big brush tip
300 g paper, more cotton it has, the better it is (minimum 25%), 18x12 cm size. Fine grain or large grain: are both good. Brands: Torchon by Hahnemuhle, cold press (hot press is bad), Langton, Arches and Fabriano Artistico, 50% cotton.
Waterbrush Pentel (big brush tip), very useful when you are travelling.
-If you want a good brush just check the tip. A good brush has a good tip.
Kelly uses "travel resealable brushes" to protect the tip. Suggested sizes: 12 or 14 number. Brands: Escoda- (spanish brand), Tintoretto synthetic or Davinci synthetic or marten.
-Brands: Windsor & Newton (not Cotman type), Old Holland, Schminke, Daniel Smith, Daler Rowney, Sennelier, White Nights (Russian brand)
Pastry or tube watercolor are both fine.
primary colors+ grey of Payne, bluish and turquoise blue
Aurelion or Gamboge Yellow
Alizarin Crimson Permanent or Rose Madder or Madder Lake or Permanent Rose
Raw Umber (optional)
Cadmium Yellow light (optional)
Cadmium Orange (optional)
Payne's Grey (optional)
Violet Dioxazine (optional)
Hooker’s Green (optional)
Metal or plastic box to put in all the colos, provided of two trays to mix the colors (Schminke or Winsor & Newton brands are ok)
HB or B pencil
Water container, folding container is perfect for travelers
Pincers to grip the paper
paper towels to clean the brush
pencil mines 0.5, watercolors pencils
Pencils, pens, e charcoals. Brands: Staedtler pencils from H to 3B, Faber-Castell china pens
-Urban Sketching: 100 postcards
Urban Sketching: the complete guide to techniques
An Illustrated Journey
The Art of Urban Sketching
There are useful to understand how many styles there are in the Urban Sketchers world. Those book will convince you that there isn't one way to do things.
Perspective made easy: a step by step guide by Lee Robbie,
Perspective made easy of Ernest Norling, art instruction,
carnet de voyage: manuel de l'artiste by Salvatore Santuccio.
Betty Edwards, Drawing on the Right Side of the Brain
Online Recurses : historic and contemporary archive of Accademia di San Luca (/http://www.accademiasanluca.eu/it/archivio_contemporaneo/informazione_e_contatti).
Jeanne Dobie, Making the color sign
Leonardo da Vinci Trattato sulla pittura (specially where he talk about the structure of the tree and vegetation ),
Art book by Hayao Miyazaki, scenography sections (for example the scenography of "My neightbor Totoro" or "Princess Mononoke")
Michael Reardon Watercolor Techniques: Painting Light & Color in Landscapes & Cityscapes
Charles Reid Watercolor Solutions: Learn to Solve the Most Common Painting Problems
Betty Edwards: L'Arte del Colore. Guida Practica all'Uso dei Colori
Jeanne Dobie Making Color Sing: Practical Lessons in Color and Design
for central perspective take a look to renaissance artists like Leonardo Da Vinci, Raffaello Sanzio and Piero della Francesca or the contemporary artist: Edward Hopper.
Degas, sketchbook (http://www.getty.edu/art/collection/objects/490/edgar-degas-an-album-of-pencil-sketches-french-about-1877/), Pablo Picasso, David Hockney
take a look to the Sketchcrawl international page on Facebook, there are artists like Mike Kowalsky, Victor Peru, Marina Marcolin, Jonathan Yule, Maneek Bonik, Richard Sneary.
Edward Seago moroccan watercolors, John Singer Sargent watercolors, Charles Reid, Andrew Wyeth watercolors.