I guess just have to make two books?
You could choose the portrait format & run the landscapes across two pages.
How is the quality of printing at blurb?
Very good. I've only had one issue with matching the dust cover with an interior pic that was caused by a printer problem. They gave me a free reprint.
I didn't care for the type + pic layouts, so I mostly made full size bleed layouts in PS & loaded them the book.
Links to two books I've done in here: Cover shot of the '69 Plymouth Road Runner 440
Is there an (imaginary) gutter in the middle of these pages?
They recommend an 1/8 margin for full size bleed pages. There is a gutter, & the "fix" is in their doc. Put the same 2X wide image into 2 pages, & in the 2 page view layout, adjust them to look right. Print a proof copy & adjust from there.
Keep in mind that I made every full page shot in PS & sized & sharpened the shot for the book. I didn't use their resizing option, so your results may vary.