He is called the master of the multi pannelled painting and i do not doubt it, because his technique is phenomenal. I have followed the career of Thielen from the very first moment i first saw a painting of his he presented to my old school. They had, if i remember well, their 75th anniversary and he had made a painting of the school facade. It looked like the real thing. great perspective, nice colors, but to me it looked very much like a clone of the best facade Carel Willink could do with textures and colors. But from that moment on his career took off with exhibition in the Netherlands and abroad. In the process he developped a style of his own and was one of the few in the Netherlands that could paint a truly large canvas. In many cases devided over multiple panels. This is not my kind of art i would like to have in my collection, but one must admire his paintings for their realistic qualities they undoubtedly have.
http://www.ftn-books.com has some nice Thielen titles available