Obligatory plug for Comic Printing UK Good for comic/non-standard sizes and finishes, great communication in general (probably helpful if you're starting out) Best to make sure you're not sending stuff to print just before a big convention though, as I've had issues w defects when they get super busy - other than that they're good about fixing any issues.
If your book is a standard size, though, A5 or A4, many digital printing facilities online can do that to a good standard and cheaper! I got a reprint of one of my first books with mixam and it was great for both price and quality. There's others too, like printed.co.uk or inky little fingers , and while I've never used DigitalPrinting.co.uk for books, all the prints I got from them have always been great.