Sorry, alema, you obviously have found what you wanted to know, but you'll forgive me for adding another comment.
MikeE said: "If you bought that book at Waterstones, it was probably overpriced."
is perfectly correct, though it does not correspond to the standard "future tense" pattern.
No surprise, because it is not a conditional structure at all. It's not a 'violation' of the 'rules' for conditional structures.
It's the verb that does the trick (subjunctive vs.true past tense with their different functions temporal/mood = attitude). Let me explain (sorry for starting from scratch).
A true conditional is 'if A, then B', and the common feature of true conditionals is that the speaker knows (is sure of) how condition and outcome are connected.
(BTW, The Thomson/Martinet Practical Grammar also uses the designations conditional type I, II, III, it's not only the DAF books).
A zero conditional, as it is sometimes called, expresses a universal truth. Pattern: present tense (if-clause), present tense (main clause). Example: If you heat ice it turns to water (taken from T/M, who don't call it 'zero'). Speaker knows that whenever you do A, B will be the consequence. Time reference: always.
Type 1: Probable condition, pattern present tense/future tense. Example: If you drop this glass it will break. The speaker is convinced that something (A) that has not happened yet, but may happen, will result in B. Time reference: from the present point into the future.
Type 2: Improbable condition, pattern subjunctive/conditional tense (with would + infinitive). Example: If you dropped that glass it would break. The speaker is convinced that action A would lead to result B but regards the possibility of A as hypothetical: the addressee is not expected to drop the glass. The same construction can also be used if the condition is not merely unlikely to become true but is contrary to fact: If I were you ... Time reference: as in type 1. The difference is in the attitude of the speaker. Note that a seemingly past tense form of the verb refers to present/future.
Type 3: Impossible condition, pattern past perfect tense/perfect conditional (would + past participle). Example: If you had bought that book you would have had a very pleasurable reading experience. The speaker knows that the condition was not true (you didn't buy the book), so the outcome is what the speaker thinks would be true if the condition had been true: the outcome is as unreal as the condition. Time reference: past. Note that a past perfect refers to the past, not a 'past before some past', as it usually does.
In parentheses, for completists: There are two 'crossover' or 'mixed' conditionals which are not mentioned in every grammar:
a) If he had betrayed me I would still be in prison is a a past event, known to be contrary to fact, that inflluences a present state of things, and
b) If I were more intelligent I would have applied for the job is an enduring (permanent) condition that influenced an event in the past (that didn't happen).
All of these conditionals, including the mixed versions, express certainty: The speaker knows that condition A is always true, could be true, might be true or was never true.
The sentence If you bought that book at Waterstones, it was probably overpriced is different. It refers to the past, and the speaker doesn't know if you bought the book there or elsewhere. It's not a conditional. Bought is a true past tense, as opposed to bought in the seemingly similar if you bought that book you would enjoy it (conditional 2 above). In the second example, bought is subjunctive (that today borrows its grammatical form from the past tense while referring to the time from the present moment on into the future), in the sentence in question it is past tense indicative and refers to an event in the past. The if simply indicates that the speaker is not sure if you bought the book there.
Compare: If I talked to her at the party, I certainly don't remember. 'Talked' refers to the past, the if-'condition' is not a condition but an expression of doubt. If I talked to her more often I would understand her better is a conditional, referring to present+future and expressing that I don't talk to her often enough to understand her well. In the first case, talked is a true past tense, in the second it is a subjunctive.
Tense refers to temporal relation, mood refers to speaker's attitude. That's why I find it misleading when grammars say you use the past tense in conditionals type 2. You use the subjunctive. For German learners, that's easy because past and Konjunktiv verb forms are (in most cases) different: Wenn du das Buch bei X gekauft hast .../gekauft hättest ....
This is the main reason why I think learners of English should know what a subjunctive is.
A non-conditional may mimick a true conditional in English, even with would in the main clause:
If you were 14 years old in 1967 ... -- no, not a conditional referring to the present but a hypothetical statement about the past, with were as a true past tense and not a subjunctive, saying nothing else than whatever follows applies to 14-year-olds at that time, no matter if you were among them -- you would have listened to the Beatles, the Monkees (if you were in the US), and probably many British or British-influenced groups. Would, in this case, is not the marker of a 'conditional tense' that follows an if-clause, it's simply the will/would-variant that expresses expectability or custom: In the mornings, we would take long walks.
That's how tricky it can get with conditionals. Don't think of grammatical rules, think of communicational function.
Hope that's more or less complete. Sorry for length.