Is there a special reason for making your book just slightly smaller than A4 - your size doesn't seem to be one thing or the other.
Margins are rather subjective. I suggest you go into a bookshop and look at books that are around your selected size and with the kind of design and content you want and see which margins seems to work best.
You wouldn't have inside bleed on books that are printed in sections.
If your book is to be perfect bound, you will also need to produce cover artwork. This will be printed separately on heavier stock than the text and maybe laminated. The flat size of the cover artwork should be twice the size of the text trimmed page size plus and allowance for the spine.
Design your book in double page spreads and supply the printer with single page PDFs with printers marks. Discuss with your printer all technical matters before proceeding.
Hi Derek, thanks for the reply. Yes, this size is requested by the client, slightly smaller than A4. I was just worried if the 15 mm inner margin would be too small, and whether full spread images would get broken up by the gutter. Thanks.
You may need inside bleed to handle the crossovers, but check with the printer. They will have VERY specific setup instructions for perfect bound.