Find next entity with ordered junction table
Your query is correct and does work. I think you can make the intent a little bit more clear by writing it in a slightly diffent way (although this is open for debate). The execution plans (on MariaDB) are exactly the same.

The comments in the middle should be explanatory:

SELECT
    sg.series_id, sg.guide_id, sg.ord
FROM
    series_guides sg
    JOIN
    (-- We get the current `series_id`, `ord`, based on `guide_id`
    SELECT
        series_id, ord
    FROM
        series_guides sg
    WHERE
        guide_id = 13
    ) AS sg0
    -- We want to be in the same series_id, but greater ord
    ON sg.series_id = sg0.series_id AND sg.ord &gt; sg0.ord
ORDER BY
    sg.ord  
LIMIT
    1 -- We just want one result
;


This will give you:


series_id | guide_id | ord
--------: | -------: | --:
        1 |        1 |   2

If you change then the guide_id to be 1, you'll get the next value as:


series_id | guide_id | ord
--------: | -------: | --:
        1 |       21 |   3

Check everything at dbfiddle here



Remarks and (simplified) table definitions:

I've defined the tables with different namings, to avoid confusions (and not to clash with the ORDER keyword, just in case):

CREATE TABLE guides
(
    guide_id integer PRIMARY KEY,
    guide_stuff varchar(100)
) ;

CREATE TABLE series
(
    series_id integer PRIMARY KEY,
    series_stuff varchar(100)
);


If guide_id must be UNIQUE (and NOT NULL, or otherwise, it doesn't make any sense) it's already a candidate key. Being the simplest candidate key, it's the one I'd use as primary key:

CREATE TABLE series_guides
(
    series_id integer NOT NULL REFERENCES series(series_id),
    -- If this is unique, make it just the primary key. It's simpler
    guide_id integer PRIMARY KEY REFERENCES guides(guide_id),
    ord integer NOT NULL,
    -- This covering index will prove useful
    UNIQUE (guide_id, series_id, ord)
) ;


Probably, you would want to have also another restriction: UNIQUE (guide_id, ord) as well, to avoid situations where your order might not we well defined.